Sandogoya Onsen in the Nasu area, Tochigi, is a hotel/onsen nearly two hours walk from the nearest road access. You can stay the night there, enjoy the hotspring and have a nice meal. This is a great overnight activity.
You can walk in from Numappara or from Nasu. Its not a hard walk but it takes a little time. We walked in and out via Nasu in very early September. It took us about 1 1/2 hours of pretty easy walking though there are some fairly steep bits. It looks like it might be a slightly shorter walk from Numappara.
We parked our car in Nasu and walked in. There is a log book there which I suggest you fill in on the way in and out, though we didn't notice it and failed to fill it in. Along the way there is a nice little fresh water stream which is marked drinking water and we filled our bottles there.
On the walk out in the morning we saw the best sea of clouds I have ever seen.
Sandogoya Onsen has a few little hotels and a public onsen (I have since heard there is no longer onsen access for people not staying there, will check this). Its really understated and nicely done. I immediately felt relaxed there. The people at our hotel could speak just a little English. They were very friendly and helpful. The cost was 9000 yen each for the two of us. We were charged 18200 at the end. Not sure what the 200 yen was for. For that price we got a nice dinner an OK breakfast and unlimited use of the onsen. There was a free pot of tea in the evening and in the morning. Our evening beers were 500 yen each.
The main hotel onsen is open one hour for men and one hour for women alternating through the day. There is a second much smaller one (3 people max!) that you can go to if the main one is off limits to you that hour.
Our room was a small simple tatami room. There were mattresses and bed linen and robes. We had each carried in a small towel for the onsen.
I would think, especially if you were from Tokyo it would be a sensational get away from it all weekend. My favorite Tochigi activity so far.