For a natural experience of a different kind, plan an excursion to the steaming pools of Harazuru Onsen. This place is famous for its mineral-rich waters.
This naturally heated water source is located in Fukuoka Prefecture. Once you've done everything you can at Harazuru Onsen, take a look at nearby Chikugogawa Onsen, Ukiha Inari Shrine, and Tsukiyomi Shrine.