Another rural supersento. This one is not too small, and has a really fun selection of baths. There's even a great cave style bath that was really fun to go in. One of the saunas is built on a fake boat in a fake river. Both of the bathing areas mimic mountain streams and is one of the best designed and decorated places in the area. The two sides, named river (川の湯) and mountain (山の湯), change so you'll have to go twice to try everything. There is some seriously fun stuff here.
Sadly, Pokapo is a car only place, but worth enjoying if you can get out here. I listed a station for Pokapo, but you’ll have to really hike it up a hill or get a taxi to get to the baths. Also, be aware that these rural supersento close early so plan accordingly.
My family and I returned to Pokapo in the fall of 2011 and I can't recommend this place more. My wife loved it. The baby had fun. I love how unique baths are here. It would be a great place to take a friend visiting from another country. I'll bet some Japanese would also be surprised by the baths here. The water is great and feels like real onsen water, which it is. The food isn't too bad either.
Update Fall 2018: Still a great bath. Can be busy on the weekends.