I really like Japanese spas and especially the Onsen which get their water from hot springs. I always check wether my ryokan hotel is equipped with such a place to relax and especially love those places if they are outdoor.
In Kyōto, Japan I went to the village Kurama, an easy 30 minutes ride away from the city center. There you can find a great outdoor pool and they rent towels if you don’t have one with you. You can sit outside in the hot water and look into the mountains.
Be reminded that Onsen are typically separated by gender – not only the locker rooms but also the pools. Which door to take is often only signaled in kanji signs. Most often the men’s room is indicated by blue color, the women’s by red.
520 Kuramahonmachi, Sakyo Ward