The onsen is right next to the Takaosanguchi trainstation. A long drive from Tokyo, but after climbing Takaosan very refreshing. Not huge, but nicely done with the restricted space.
Just a small part outside, this sento is at the 2nd level of a building. Not much green, not big. 1080円 for 1 hour (you bring your towel), 1700円 for 5 hours including towel set. Special rates on some days, I payed ~1200円 for the 5 hours ticket.