For an unique natural experience, plan an excursion to the bubbling pools of Chikugogawa Onsen. This place is famous for its mineral-rich thermal waters.
This naturally heated water source is located in Ukiha. Once you've explored everything you can at Chikugogawa Onsen, have a look at nearby Harazuru Onsen, Ukiha Inari Shrine, and Tanada Shinsui Park.