I'll start with some important infos, then tell you what we did and then give some recommendations for sightseeing, for skipping parts of the trail and what to look out for before renting a bike.
• Do not do this when it's above 30°C. It's ~75km with ~500m elevation gain and not a lot of shade.
• Rent the bike well in advance
• E-bikes can't be rented overnights
• Skip the first bridge from Onomichi to Mukaishima. It's 100m hight difference and the ferry is only about 200¥.
• The recommended route is very clearly marked and also well maintained.
• Most POI (points of interest) are a bit off though, so decide beforehand what you want to see.
We were two people, I took the battery-assisted bike and my SO the normal one. On flat terrain the normal bike is a lot faster, since mine only had 3 gears, but it pays of uphill.
We stayed in Onomichi, so we could start early and also because it's a very nice little town. For the most part, we stayed on the recommended route. First stop was the Kosanji temple on the 3rd island. Very cool (but you have to pay an extra 200¥ fee for one of the buildings). Then we ate in Setoda (where the temple is) and drove on to the next island, Omishima, where we took the ferry to Rabbit island. We could only stay an hour, since the last ferry back was around 16:30.
We slept on Hakata Island and visited the Kaizoku Museum on Oshima and chilled on the beach a bit. We returned our bikes in Imabari.
Tips:
Now, the first two islnds have some interesting POIs but if you don't want to visit them you can skip the whole islands. They have some steep hills and are not that scenic.
Kosanji temple is really impressive, since it also includes the Hill of Hope and the Cave of 1000 Buddhas.
Next to the city is a very nice beach, so you could spend the rest of the day there.
The ferry to Rabbit Island is 6km distance from the recommended route on Omishima. If you decide to visit it, check the ferry timetable and buy rabbit food beforehand. Some will let you pet them but don't try to pick them up!
The last island, Oshima is again not very interesting and furthermore also very hilly (on the recommended route). The Kaizoku museum there is cool, even though they did not translate a whole lot of the info.
Renting bikes is easy online. If you are moderately active, the normal bike will work fine if you take 2 days. The battery assisted bike is great if you feel unsure about going uphill and can also be rented for multiple days. It is sadly very slow on flat terrain as it has only 3 gears.
Rented from here:
https://visitshimanami.com/bike-rental/
Edit to add link
by suzume1310