Again Yarigadake, this time not on the ridge but down in the valley, where there is Yumata Onsen. To reach it one has to go to Nanakura in Nagano Prefecture, park the the Nanakura parking and take a taxi to the Takase dam (or walk). From there it is a nice 3 hour hike to the Onsen.
The place to stay is a simple but nice hut, and many people also put up their tents around the place. The hike itself is very easy, accessible with kids.
The Onsens in the area are right in the river, or next to it, below the hut and above the hut. Including some beautiful forms, one even national heritage (as far as I understood).
Thanks to Masumi for teaching me the place.