In the midst of the mountainous Okukino region of 'secret onsen', you'll find Kaniyu Onsen, a rustic ryokan with milky sulphuric waters in its multiple outdoor baths, a few of which are mixed bathing. To get here from Nikkō, take the Tōbu line to Shimo-imachi, change to a Kinugawa Onsen–bound train, then board the bus to Meoto-buchi (¥1540, 1½ hours, four daily). From here, it's a gentle 1½-hour hike up a beautiful river valley past several waterfalls.
You'll need to leave Nikkō before 9am to make it to Kaniyu and back in a day, and you'll still only get about one hour for the onsen. If you'd prefer to take it at a more leisurely pace, plan an overnight stay. Either way, check the latest transport details with one of Nikkō's tourist offices before setting off.