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).

Myoga-ya is a 340 year old Onsen Hotel in Oku-Shiobara.  It has been run by the same family for more than 17 generations.

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.

All the rooms are Japanese style tatami rooms and the food is traditional fresh seasonal food, for the main part sourced locally.  The hotel staff are wonderful, and despite not speaking English, are happy to have overseas visitors..

In the lower level of the hotel is a small gallery with paintings, pottery and some old photos demonstrating some of the history of the hotel and region.I believe you can just have an onsen at this hotel, but I think the fee is quite high. On the other hand, we shared rooms (2 to a room), and dinner, onsen, the rooms and breakfast was all inclusive at around 10,000 yen each. Very reasonable, and a truly memorable experience
Myogaya website