Jigokudani Rotemburo. Here’s the bit about that monkeys that I got off the internet somewhere:
Groups of wild Japanese monkeys are living in Jigokudani Yaen-koen, which is located in the mountains in Nagano. You can see these famous monkeys all over in the park. They are used to being around people and living as if they are humans.
There is an outside hot springs that was made just for the monkeys. In winter, you will see them soaking themselves in the hot water while the snow falls on their heads. Somedays they stay up in the mountains, but hopefully you will be able to see them bathe or swim in the hot springs.