Jouzankei is an onsen village an hour from Sapporo Station by bus and is located within Sapporo city limits. You didn't mention your budget, but Suizantei Club Jozankei (翠山亭倶楽部定山渓) is a very nice ryokan with in-room onsen baths (some indoors, some outdoors depending on the room).

You might also look at some of the options discussed here.