Right in the middle of Yunomine village, this UNESCO-listed hot spring sits inside a tiny wooden shack built on an island in the river. You'll buy a ticket at the nearby sento and have the minuscule bath — barely big enough for two people — entirely to yourself for up to 30 minutes. The water changes colour throughout the day, shifting from milky white to blue to green, and the steaming little cabin feels like something from a fairy tale. It's the only bathing onsen in the world with UNESCO World Heritage status, and at under 800 yen, possibly the most valuable half-hour in Japan.