Kumamoto Prefecture (熊本県, Kumamoto-ken) is a great place to go to if you’re looking for hot springs; they’re everywhere, including the popular Kurokawa Onsen. Be sure to visit Kumamoto Castle, one of Japan’s top castles, while you’re there as well.
If you like nature, Kumamoto is also a nice destination; Suizenji Park is perfect for photographs, while Mount Aso is one of the world’s largest active volcanoes. Take a look at the unique Tsujun Bridge for some architectural magnificence.
One popular activity in Kumamoto is riding the scenic Steam Locomotive Hitoyoshi; make sure you get reserve seats well in advance, as tickets sell out surprisingly quickly. Kumamoto is also famous for its raw horse sashimi; if you can't muster up the courage to try that, have some tonkotsu (pork-bone broth) noodles instead.