When L was still living in Yamanashi Prefecture, and I was living in Tokyo, I used to visit her whenever I can. On one my visit, L found and interesting onsen near her place : a Wine Onsen. The two of us being onsen addict decided to check it out. The onsen is actually one of the feature of Hotel Hatta.
The onsen contains Red wine extract giving the onsen its red color. There is no alcohol content though so it is safe for kids to get in also. Walk-in guest are allowed to use the onsen from 10:30 -17:00 only and it cost 1480 yen with face towel. Soap, shampoo, and other amenities like toothbrush and comb were also available.
The hotel have several other onsen too but L and I did not know about this till we were leaving. The wine onsen was located on the top floor of the hotel and it was glass around so on a clear day you might be able to see the tip of Mt. Fuji.
Nearest station is IsawaOnsen(石和温泉) station.