Nakaya Ryokan

Nakaya Ryokan is in Yubiso onsen, part of the Minakami hot spring area in Gunma. Its two open-air baths have no roof, so you get a view of the mountains by day and stars at night, which is where the ryokan's name, Tenku no Yu (天空の湯, "sky bath"), comes from. There's also a gender-separated indoor bath, plus a private bath you can reserve, kept dim for a quieter soak. The newest room type comes with its own small cave-style bath, fed straight from the spring without dilution, with a warm tub and a cooler one to move between. The spring water itself is mild and smooth, without a strong mineral edge. Reviews describe an easygoing, unpretentious inn that goes out of its way for families with small children, from in-room dinners to flat tatami rooms suited to toddlers.