I've not been to Shikoku, but have been to Kyushu several times.

In Fukuoka, the dozens of nighttime food carts (yatai) that set up along the river near Nakasu are nice for enjoying a meal a dish or two at each stop. I enjoyed visiting Kumamoto Castle (it was at the height of cherry-blossom season though), and Kagoshima was interesting incl the volcano (sort of like Naples I guess)