Tennen Onsen Hinoki No Yu

Tobira Onsen, deep in the mountains of Matsumoto. You drive 10 km up a narrow mountain road; there's one high-end hotel nearby, but that's not our destination - we go a bit higher. Entry is 400 yen (!!). No soap. The water smells strongly of sulfur, which it has plenty of. It's also alkaline (pH 9+). The baths have gensen water that you can even drink (tastes awful, but still cool). The best part is obviously the view from the rotenburo - forest and mountains, it's stunning, you're literally bathing in the forest. The water isn't hot, you could sit there for hours. Getting there without a car - no idea how, but if you can manage it, this is a mega-top spot and a strong recommendation to visit.