Karuizawa Sengataki Onsen is a public hot-spring bath in the woods of Naka-Karuizawa. The appeal is simple: mineral water, a quiet garden setting, and an outdoor bath where clear nights bring a good chance of seeing stars above the trees.
It is a low-key stop rather than a destination resort. Bring your own towel or check rental options, and treat it as a restorative break after cycling, walking, or sightseeing around Karuizawa.