Japanese onsen (hot spring) Ryu Doy, You can enjoy 18 kinds of private baths, moreover no time limit.

Secret hot spring
Hotel Ryu Dou in Gunma prefecture has 18 kinds of baths, more over all of them are private with no time limit.
There are also large outdoor baths.

Ryu Dou official.
https://www.ryuudou.com/