Wonderful place in the forest where you can make a hot bath. The hot water comes out of bore holes in several places besides the river. There are some places where it comes out naturally from rock fissures.
This has only been added to Nikko area Japanese hiking map in the last few years. There is quite an easy hiking trail there along the river from the bus stop (Heikeidaira [平家平温泉] ) just before Mie-oto-buchi onsen ... on the bus from Kinugawa onsen station.
There is nowhere to stay so you need a tent etc. It is possible to go there and come back in 1 day if you start early from the bus stop. It is not possible though in 1 day from Tokyo by public transport.
The path along the river has small bridges but sometimes washed out of places by floods so you might have to take your boots off and walk across the river a few times - its not deep, maybe 10cm to 30cm.
There are two main spots;
The first one, and probably the easiest to make a bath is Hirokawanoyu [広河原の湯] about 1.5 - 2 hours along the path. It is a wide flat riverbed area with hot water gushing out of a hole. It can be directed into a pool with a pipe there and mixed with cool water to get the right temperature. There are usually several large blue plastic sheets there for lining the pool and usually a bath already prepared from previous visitors.
There are a couple of other places along the path from there and after a further 40 minutes there is a waterfall with a large yellow steaming sulphur vent - its a desiganted natural treasure of Japan!
This is Funsento [噴泉塔].
It is possible to make a bath in one of the pools below the waterfall.
There is another nice pool above the walerfall but the route is hard to find ... better join Outdoor Club Japan and go there on one of their trips.
Outdoor Club Japan
Check also this guide
on Okukinu area.