Mikurigaike Onsen

You have probably already been here since the route is popular, but I really liked Mikurigaike Onsen みくりが池温泉, the highest altitude onsen in Japan (~2410m). I went this past weekend. In the women's bath, there are two small baths, quite hot, one hotter than the other (about 40-42 degrees Celsius), with water rich in sulfur, coming directly from a nearby source. I highly recommend the main onsen restaurant; the food is very delicious, and the green tea and sake are some of the best I've tried in Japan! You can stay there as well; there are rooms and a good souvenir shop.