One of Tokyo’s few true onsen, this chic spa complex, renovated in 2017, relies on natural hot-spring water from 1700m below ground. There are indoor and outdoor baths, saunas and a bunch of add-on options, such as akasuri (Korean-style whole-body exfoliation). It's a fascinating introduction to Japanese health and beauty rituals.
An extra ¥865 gives you access to the Healing Baden area, with even more varieties of saunas and a lounge area styled like a Balinese resort; here men and women can hang out together (everyone gets a pair of rental pyjamas). There are lounging areas too, with reclining chairs. For an extra ¥1950 you can overnight here.