Kita Onsen - map location must be one of the great hot springs in the Nasushiobara area. It has a beautiful location, good water, lots of history and style. You can stay at Kita onsen overnight or you can drop in for a day onsen. Its also one of the entry points to Nasu park.
I don't know the history of the Kita onsen building. But I know Kita onsen opened in 1696! The building is really old. Sometimes I think Japan defines rustic, dilapidated and the boundary between differently. But Kita isn't about faded carped and rusty metal, its definitely rustic and has heaps of style. If you go there for a day onsen the entrance area will be full of friendly walkers flopped around the firepit. Its a great place.
There are a number of pools in Kita onsen. First up there is a giant one out the front. Its not my favorite one due to the fact the arrival path goes right by it and a number of rather crappy additions (like the metal slide). Never the less I've sat there looking up at the mountains under moonlight with a group of friends and can say its very nice. The best pool is the roten up the back. Good views of the river and the strange concrete structures it flows over. The water here is good, flow is great.
You can stay at Kita overnight. I recommend it. The great old building made of heavy timber has a great atmosphere. The staff are very nice. Its the the lower cost, simpler end of the ryokan experience though. Dinner is simple and eaten in your room. Its good. Breakfast also simple and eaten in a dining hall. When we went we had it all to ourselves pretty much. Its some time since I went, but it cost 6 or 7,000 a head for 4 to a room for our party. If you're looking for shiny, elegant and expensive then Kita is probably not for you.
Local and Access:
Its a fairly short walk (5 or 10 minutes) from the carpark to the onsen. It can be a bit treacherous in winter. The location is fantastic at the bottom of the mountains, next to the river. If you are on foot, and you walk up the carpark and then keep walking up to the main road, across the road there is a track that will take you all the way up to Omaru.