Another vote from me for Shikoku, or at least as part of something. I'll paste here what I wrote in the other thread.

Starting from Kobe (or Osaka), rent a car and make your way to Kyushu via Shikoku. Start by crossing over to Awaji-shima and optionally drive along the beach instead of via the expressway. Then go to Shikoku, eventually driving to the other end. At the end of Shikoku, catch a ferry to Oita/Beppu area. From there either head towards Fukuoka, passing through Yufuin, or go the other way (South) towards Miyazaki. Return by driving through Hiroshima, Okayama, Kurashiki areas, etc. There would be numerous variations and branches off of that, especially if driving. Downside would be potentially lots of driving each day, so maybe choose a couple of landing spots and do day trips from there.