GoNative
yes you`re right, i totally forgot about Toyako because there i stayed in the Youthhostel wich didn`t have an Onsen. The Area around Toyako is very nice, you can also see some vulcanic action and you can walk through a Area that was destroyed by earth movement wich is quite spectacular with broken streets, destroyed houses and steam here and there. The lakeside is also nice and i think there`s a firework there every evening in summer. I haven`t been in the town itself so i can`t say anything to that but here is my
post to the suroundings.
It is a bit further away from Sapporo than Noboribetsu but i think its better to go there. Haven`t been to one of the other 2...