I've used Fukuoka as a hub and did day trips to Nagasaki, Kokura/Mojiko, and Chiran/Kagoshima.
Hiroshima and its surrounding area is pretty expansive to just limit to a day trip, IMO I'd actually use it as a hub and go see places like Kure, Iwakuni, Miyajima, etc.

Nagasaki trip was cut short due to rain, wanted to go see Dejima but didn't want to walk around in the weather so I kept to indoor places like Atomic bomb memorial, Siebold museum, food places along the tram line, etc. I'd like to go back again and go see other areas like Sasebo and the Nagasaki Bio Park.
Mojiko has a nice shopping area in the retro district, but the flea market in unfortunately closed in December. Also went to the rail museum there and found good sushi places for food.
The Kamikaze pilots memorial and the town museum with the showa era reproductions took most of my time in Chiran but I was also able to fit a stop at the Kagoshima aquarium before catching the shinkansen back to Hakata. I would have gone to Sakurajima if I had an extra day there.
Kumamoto castle was closed when I was there due to the earthquake damage so I passed intending to go back when it reopens