Noboribetsu - similar to Beppu in that there are many onsens (hot springs) in the area and also a lingering smell of sulphur. Much more to see here than in Beppu though, with Noboribetsu having a nearby active volcano (Mt. Hiyori) and mini calderas that are fun to look at.
Jigokudani (Hell`s Valley) - remains of a volcanic crater created by Mt. Hiyori