Kusatsu is a high mountain onsen town in Gunma, famous for hot water that arrives in town with almost theatrical force. The centre is the steaming Yubatake, where mineral-rich water pours through wooden channels before being sent to baths around town.
This is not a quiet spa hidden in the hills. Kusatsu is lively, walkable, and strongly built around its hot-spring identity: people stroll between baths in yukata, steam drifts across the streets, and the sharp sulphur smell reminds you that the volcanoes above town are still part of the story.
Kusatsu works best as an overnight stop, but it clears a spare day easily if you are already in Nagano, Takasaki, or the northern Kanto mountains. See the Yubatake, watch the yumomi water-cooling performance at Netsunoyu, then walk west to Sainokawara Park for hot streams running through the valley.