Directly south of Sapporo and surrounded by soaring volcanoes, Shikotsu-ko (支笏湖) is the second-deepest lake in Japan and renowned for its clear water. While it is 250m above sea level, its deepest spot is 363m, 113m below sea level. You'll need your own wheels to get here, but it's a superb spot for independent exploration and excellent for campers.
Shikotsu-ko Onsen (支笏湖温泉), on the eastern side of the lake, is the only town. This compact little resort village has some nice short walks, including a nature trail for birdwatchers. Sightseeing boats head out onto the lake and there are rental bicycles, boats and canoes.