I second Noboribetsu. The "Hell Valley" (Jigokudani) was certainly something memorable (basically a steaming, rocky valley with streams of boiling water and little bubbling geysers - very close the main part of Noboribetsu), you might even be able to check out Mt Hiyori as well.
Check out this guy's videos (17, 18 and 19) >>
Oyunuma and Mt. Hiyori in Hokkaido - AOL Video