60km. 6 islands. 5 bridges. Good 1 day adventure. Start and end the day at Onomichi. I've listed my recommendations for the trip here. If you need some quick tips on the trip without any spoilers, just read this bit!

My Recommendation

Onomichi - Imabari (70km)

This is my recommendation for a couple of beginner/intermediate road cyclists who aren't on a super tight budget. If you are a beginner cyclist, it might be better to hire a cheaper hybrid bike, with the option to abort earlier and catch a bus home if you need.

Book 2 nights accomodation at Onomichi Cycle Hotel. Make sure you include breakfast https://www.onomichi-u2.com/en/#hotelcycle Approx ¥9,000 per person per night.
Catch a train to Onomichi. Make sure you arrive by 5pm if you want time to pick up your bike from Giant.
Pre-book a bike hire at Giant (http://giant-store.jp/giant-store-rentcycle/en/index.html). Note that you'll need to pay a ¥3,000 excess, to drop off the bike at Imabari. (although you might be able to find a bus with bike racks to bring the bike back to Onomichi).
Start the ride by 9am
Tire change gear, lights, helmet (bike shop will lend you these in the hire price). If you are bringing clipless shoes let them know what pedals you want fitted in advance.
There are lunch stops along the way and a couple of lawsons convenience stores. Unlike most places you might be used to as a tourist though, there are far less convenience stores and vending machines than you might expect, so bring a drink bottle and a backpack with some snacks if you need
camera + phone charger
This ride is supposed to be 60km, however my Strava says 78km, go figure.
The ride is coastal with lots of bridges. You will face plenty of wind. I've read that there is generally more headwind when traveling south. This was consistent with our experience. I'll never forget the headwind on the final bridge.
This map has all the info you need: http://www.go-shimanami.jp/download/cycling/cycling_map_en.pdf? (4MB download)

Onomichi - Imabari - Onomichi Return

This is a fun option if you don't need to stop at every bridge and take selfies. It will be more physically demanding, but perhaps more rewarding (and cheaper).. If I ever do the ride again I'll certainly do this option.

Make sure you book a carbon or premium carbon bike from the Giant store, and pick it up the night before
You might not need to worry about paying for breakfast on Day 1 at Hotel Cycle (not sure what time breakfast starts).
Try this in the summer months where the light stays up for longer

Our Adventure

I had previously decided that any good holiday should involve snow sports, climbing, and amazing noodle soup. C and I decided to mix it up and add cycling to the list on this trip. It wasn't the first thing we thought of to do as an extra activity in Japan, however it turns out that one of the most amazing 'friendly' bike trails in the world is in the middle of Japan.

I say 'friendly' in that it's a pretty easy ride, no super tough climbs that reward you with European style mountain vistas. But what you do get is a predominantly off-road ride of well sign posted well paved paths, with incredible sea views and amazing bridges.

We did this ride at the start of Feb. A colder month, but possibly one of the better times to do it, until the final bridge, on the whole ride we only passed 2 other cyclists. Not warm but not too cold either.

Planning

Like all things in Japan, planning was harder than we expected due to the language barrier and lack of detailed info on websites. Where should we stay, where to we hire bikes from, and how do we get from one town to the next after we drop off our bikes.

Our schedule was

Tuesday, Land in Osaka at 1pm.
Tuesday 6pm our cutoff time for arriving in Onomichi to hire bikes before the hire store closed
Wednesday 6pm get our bikes to the hire store in Imabari
Midnight: Arrive in Hiroshima

Our original plan was way too ambitious, we planned to ship our luggage to Hiroshima after checking out. And in Imabari we would find a way to catch a ferry to Hiroshima, carrying 1 night worth of clothes + toiletries with us for our first night in Hiroshima. Thankfully we abandoned this plan at the very last minute (literally 20min before heading off, as we ate breakfast before departing).

So instead this is what we did...

Osaka to Onomichi

Shinkansen from Shin-Osaka to X (JR pass should cover this). Then a local train to Onomichi. Roughly a 2 hour journey.

You certainly know you arrived at the right place when you get disembark the train and see this sign.

If you arrive in the day time, makes sure you head to the tourist office to pick up a timetable. We could not find information anywhere online about how to get back from Imari to Onomichi. These guys had a super simple timetable.

The Onomichi U2 building is a quick 5 minute walk from the station.

The building is pretty huge. At 9:30pm when I arrived it was dark and I missed the entrance. Around half way up the road facing side near this light.

Onomichi U2 Building

Even by Japan standards of cleanliness and hospitality this was an amazing hotel, I'm biased I guess because this place had a cyclist vibe to it (obviously!). The room itself was clean and functional, not huge but big enough for us to prepare for our ride. What made it amazing though was the convenience of the building the hotel was in. A bike hire store, a super hipster restaurant (without the hipsters!). Lots of nice fresh bread in the morning to fill up on the carbs. Dinner was great too, they did western style dishes, and pulled it off pretty well. We nearly wanted to come back and eat here again after we had checked out.

This hotel is so good that I'd say it's an part of your Onomichi bike ride experience. Make sure you book well in advance, because they don't have too many rooms and I would say they are in demand as more cyclists find out about this place. Their website is here: https://www.onomichi-u2.com/en/ Note that they charge per person, not per room. This is a little strange, C and I could have actually booked 2 separate rooms for the same price. We paid ¥9,000 per person. Go for the booing which includes breakfast. It's worth it, even if just for the convenience.

Bike Hire

We hired our bikes from the Giant store http://giant-store.jp/giant-store-rentcycle/en/index.html From what I have read, it can be far cheaper to hire from elsewhere (10x cheaper!) and some hire places let you drop bikes off at different locations for free.

We came all this way though, so we wanted nice light smooth road bikes. They will provide you with locks/lights/helmets/tire pumping gear. Just bring along your padded shorts. If you can bring your bike back to the same store by 6pm you'll save ¥3000.

Email the bike store in advance (a few weeks in advance would be good). Let them know you are staying at Cycle Hotel. if you give them your rough measurements and bike preference, they can set a bike aside for you. You'll need to pick up the bike by 6pm the day before, give yourselves some time for bike adjustments (and to take the bike for a quick spin up and down the road outside, to make sure everything is in order). Once you have your bike, you can wheel it over to your hotel room, in the same building :)

Ferry

The ride starts 5 minutes from Onomichi U2 at the ferry terminal. From memory there was some small coin charge (perhaps ¥200). Ferry runs 3 times per hour, a 5 minute long trip.

The Route

The route itself doesn't require too much explanation. The route marking was amazing. It was virtually impossible to get lost. You'll see these markers painted on the ground everywhere. If you ever did get lost - just look out for the next bridge and ride towards it! The markers are for the Onomichi-Imabari ride. There are alternate rides though which are slightly longer and might bring you to some of the higher island peaks.

We've listed our Strava route below. No records set here, but gives you a little bit of an idea of elevation and distances.

Under the Innoshima Bridge. 1.2km of straight line makes for some nice vanishing points

Lunch

There is a perfect stop for lunch half around half way along the ride, at 'Setoda Sunset Beach'. It feels like in the summer time this place would be like Bondi Beach. However in Winter, it was absolutely dead, we were pretty worried there would be nothing open, fortunately we were wrong, a shop which usually caters dozens of leisure cyclists at a time, still there just for us. It feels like they don't really open to turn a profit in winter, but as a community service to prevent cyclists from starving. Lunch was around ¥800.

Scenery

Make sure you do leave time to stop along the way. There are plenty of nice vantage points before, and after each bridge. Some times it's fun to chill out and watch the ships float past. Along the way (just before some signs labeled 'cycle sanctuary' there is a lookout you can climb up onto, somehow we didn't grab any photos of it, but it looks like something out of a low budget 007 movie. There are probably plenty of other gems along the way which you'll only notice if you stray a little further away from the blue line.

Kurushima-Kaikyo Bridges

This bridge is epic. Right now it's the longest suspension bridge in the world. Stunning views, amazing leadup. Doing the ride north-south means you reach this bridge last, I feel this is the way it should be done, doing the ride North-South would be rather anti-climatic. There are some good photo ops on the bridge as well (as your friends cycle up the loop). We ran out of time though, the sun was setting as we rode up the ramp to the bridge. In theory we still had around 1 hour left, the time disappeared very very quickly though. We battled headwind on the bridge, and we were a little anxious about how late it was. Next time we need to arrive earlier so we can enjoy the view of the islands down below.

Imabari Drop Off

One thing we didn't properly mark was exactly where the store at Imabari was. Since we were running late, we freaked out. Fortunately it's pretty much right under the station. Impossible to miss. I've marked it on the map below.

You'll need to return your bike at 6pm. We faced way too much headwind on the final bridge (and took too many pictures). So we arrived late, around 6:10pm. They could have charged us excess, but they were nice and let it pass. Lucky!

We didn't stay around Imabari as it was pretty late/dark/cold. While we were waiting for our bus however we did find a toilet on the southwest side of the station. Also a trusty bakery inside the station. Make sure you grab some pastry for the bus ride home. (But don't leave any mess on the bus!)

Bus to Onomichi

We wish we knew more about this option in advance, it wasn't very clear from info online. The hotel receptionist gave us a little info (however it was slightly out of date), they directed us to the tourist office at the train station, where they gave us this super handy paper timetable.

At the Imabari stop we solved another logic puzzle using time-table and map clues, to figure out we should be standing at queue '2'. The bus which picked us up was a greyhound coach style bus. It wasn't cheap, around ¥4,000 from memory. You have to change bus half way at the "Innoshima Ohashi Bridge". If you don't speak Japanese, it will help if you show the driver the timetable as you get on the bus, so that they understand your intentions.

Apparently some buses exist which can transport bikes. Which would be a good way of getting your hire bike back to Onomichi without paying the ¥3,000 'other location drop-off' fee.

Back to Cycle Hotel

Our bus actually missed it's connection and we ended up at a train station. That's a story for another time, but we didn't end up at U2 hotel until 9:15pm. Super late at this point! We ran to pick up our bags and had the hotel call us a cab, we cabbed to the closest Shinkansen station, and then we were off to Hiroshima. By the time we arrived in Hiroshima it was around 1am. We were exhausted, but proud of our achievements! Last minute plan changes and a few dramas along the way, but we made it!

Next Time

We were pretty happy with how efficiently we did this (staying at Cycle Hotel for just 1 night, coming from Osaka on Tuesday and being in Hiroshima by Wednesday night), however for a more relaxed time, it would have been better if:

We had known about the return buses in advance (rather than a ferry being the only option)
We booked 2 nights at Cycle Hotel
We started the ride at 8am rather than 11am

Now that we have done the ride once, I'd certainly come back and do it again, next time we will do twice the distance. We've done a few 100 mile rides now, so the distance wouldn't deter us. We've also taken enough photos, so the next time it would just be nice to focus on maintaining momentum, just stopping for lunch at Imabari. I'd stay we will start at 7am, with an aim to be back for the bike store drop-off cutoff at 6pm. The downside will be missing breakfast!