I've been to that onsen when I was living in Hokkaido as it is run by my then Hokkaido University Sensei's wife. The place is really nice and there is plenty of hiking opportunities around. I stayed overnight there, in a ryokan style room with my sensei's students, and we also enjoyed a nice meal. However I'm not sure the "sleeping overnight" option is available to every body. It's more like a mountain hut style house. But I'm not sure they're doing it anymore.
Anyway, the rotenburo on the outside is nice, and your skin will remember it.