Noboribetsu is a small town in Hokkaido, of about 52,000 people. It is about 1 hour by train from Sapporo. It is the home to the Onis (demons), as well as the natural volcano known in English as Hell Valley. If you want to stop by in August, there is a Hell festival in which people dance in strange costumes before the Devil comes down pulled by demons.
There is also a ninja park set in the Edo period style, and great hiking and natural Hot Spring.
The downside is that in the winter its very icy, so watch your step. Also, there is a terrible bear park were you can see/smell bears that are locked away in tiny cages.