If the idea of sitting naked in pools of hot steaming water surrounded by pristine forest appeals to you, then head to Fukiage Roten-no-yu. The turn-off, 2km before Tokachidake Onsen on Rte 291, is signposted. There's a parking lot, from where it's a 200m walk through the woods to the springs.
There's nothing there except two hot pools; the one higher up is hotter than the other. It's not for the shy: strip off and hop in! There's no charge…and this place is konyoku, meaning men and women bathe together. You can take a small 'modesty towel' if you prefer.