I just spent the weekend in zao, lovely little town.

however the OP appears to be looking for somewhere between Osaka and Kyushu. Which says something on a seto inland sea island to me. Unfortunately I don’t have an answer other than some things on the Hiroshima coast but something in the setonaikai national park would be lovely. If you are feeling sporty you could rent bikes and ride the Shimanami Kaido across the bridges through the national park. Has been on my to do list for a while.