Onomichi is the Honshu end of the Shimanami Kaido, the 70 km cycling route that crosses islands and suspension bridges to Imabari on Shikoku. Even if you do not ride the whole route, the first ferry hop to Mukaishima and a short island ride give a taste of why cyclists talk about it with such affection.
Bike rental points cluster near the station and port. Strong riders can make the full crossing in a day, but a slower two-day trip leaves more room for beaches, citrus groves, and temple detours.