For a natural experience of a different kind, organize an outing to the bubbling pools of Subasiri Onsen Tenkei. This spot is famous for its mineral-rich thermal waters.
You'll be spoiled for sightseeing choice when in Oyama. Subasiri Onsen Tenkei is a great place to start here, but there are plenty of other fine attractions to consider. Fuji Speedway, Lake Yamanaka, and Lake Yamanaka Community Plaza Kirara are close to the naturally heated water source and will fill in your day nicely.