For an unique natural experience, make your way to the steaming pools of Hirauchi Kaichū Onsen. This place is famous for its mineral-rich waters.
When you've spent a bit of time at this naturally heated water source in Yakushima, why not check out Mt. Mocchomudake, Onoaida Onsen, and Snake's Mouth Waterfall? They're not far from Hirauchi Kaichū Onsen and also popular with travelers.