Kuma no Yu is a free outdoor onsen near Rausu, with hot mineral water in a wooded setting. It is simple rather than polished: bring your own towel, do not expect soap or changing facilities beyond the basics, and follow local bathing etiquette carefully.
There is a separate women's bath and a mixed bath. The mixed pool is partly screened by trees but still close enough to the road that privacy is limited, which is part of the rustic character of the place.