I have not been there but I have gotten into a curious spa.
There is a big spa amusement park called Yunessan near Tokyo.
There are various hot springs.
For example, coffee bath, wine bath, and green tea bath and more.
On the special event, I could take a hypersaline bath like a Dead Sea.
I was really buoyed up by the saltwater.
However, the salt hurted my little wound.
Anyway, it was fun and a good memory.