Shiobara Onsen Village is an onsen area south of Nasu, with eleven different water sources providing many different mineral content baths (clear onsen, milky onsen, salty onsen, iron onsen and many others) making it well worth a visit. The resort is over 800 years old and is a charming, rustic area with many onsen and hotels where you can stay overnight or just stop by for a casual onsen (mostly for a few hundred yen).
The hotel is about 40 minutes from Nasushiobara by car and surrounded by forests, nestled on the edge of the Hokigawa river. There are a number of baths, at least five of them outdoors on the edge of the river.
The river edge baths are over 200 years old, so be prepared – there’s no sophisticated changing/showering areas. Most of the outdoor baths are mixed bathing, but there are some “ladies only” times and also a ‘ladies only’ bath by the river.