Ryuudo

Ryuudo sits at 800 meters in a secluded valley of Minakami, Gunma, with 18 individually named private outdoor baths scattered across the grounds. Every bath runs on fresh spring water (kakenagashi) and is locked from inside, so you just grab a wooden nameplate, pick whichever bath is free, and stay as long as you like. The water is clear and smooth on the skin. Baths range from a large riverside rotenburo and a cave bath to a cypress tub, a sauna-equipped bath, and rooms with their own private terrace pools. A stamp rally lets you work through all 18. The shared rest building, Yu-yu-kan (遊湯館), is a farmhouse from the Meiji era, with an open hearth where you can warm up between rounds.