There is a fair bit to see and do in Onomichi.
Here is a good summary.
Built high on the hill overlooking the city, the temple is over 1000 years old. You reach it using a cable car which gives great views too.
Cablecar station
Onomichi at night
One option worth exploring is the tourist train Etsetora which looks like fun! The outbound from Hiroshima runs 09:32-12:35 which is a bit late for us, but the return is 14:38-17:25 which is just fine. The train does not run every day but might fit Garry's schedule (not mine).