Yamakawa Sand Bath Hot Spring "Sayuri"

Here they bury you in warm sand on the beach (bliss), and after the sand there's a shower to rinse off (soap and shampoo are strictly prohibited) plus two indoor baths to soak in. The full set costs from 1500 yen (with your own towels) up to 2300 yen. If you don't want the sand and just want the janky baths, you can get in for just 300 yen. The larger bath is 42.4°C, the smaller one is 39.3°C, clear water. From the pipe layout on the walls it's obvious they mix cold and hot water right at the baths.