Kaniyu (Okukinu Onsen)

Kaniyu is the biggest of the four inns at Okukinu, a group of lodgings deep in the headwaters of the Kinu River in the mountains of Nikko, where private cars are not allowed. The public road ends at Meotobuchi. From there you either ride the inn's shuttle, about 25 minutes, or walk a forest trail for 90 minutes to two hours.

Getting to Meotobuchi itself takes a bus of about 95 minutes from Kinugawa Onsen Station for 1,570 yen, and there are only four a day. The inn's boast is variety: five sources with different water, which you can compare one after another, including the cloudy, sulfur-scented water the valley is known for. The outdoor baths are where the clothing question comes in.

The first rotenburo is for women only; the second and third are mixed. The inn's terms on Jalan say you may bathe in the mixed pools wrapped in a plain single-color towel, but swimsuits and bathing wraps are not allowed, so the choice is nude or towel. Indoor baths are split by gender, and a private outdoor bath can be reserved free on arrival.

Rooms start around 24,800 yen for two. The main obstacle is booking: day visits need an advance reservation made in Japanese, and the shuttle runs only a few times a day.