Lake Toya (Toyako in Japanese), is part of Shikotsu-Toya National Park and is about an hour and half from Sapporo by train. Lake Toya is really famous for hot springs and also is home to Japan’s most recently active volcano, Mount Usu, which last erupted in 2000. On a nice day, Lake Toya offers a spectacular view of the lake and the islands in the middle of it, but like most of the rest of my trip to Hokkaido, the weather was once again cloudy. The lake is pretty large so I recommend renting a car or bike as public transportation is not so frequent. For the best views of the lake and surrounding area check out Mt. Usu Ropeway.
There are a few nice things about Lake Toya that I really liked. For one, there are all these (sometimes) interesting sculptures all around the lake (as you can see above). Second, every night in the summer there is a small festival at night where there are fireworks over the lake and also a small traditional dance and taiko drums. Also if you are into outdoor activities there is plenty of hiking, fishing, camping, boating and other water activities available.
For more information please see Japan-Guide.com.