Onuma Quasi-National Park
Ōnuma means "great marsh", a fairly apt description of this placid lake. Divided in two by a thin strip of land, the smaller western portion of the lake is known as Konuma (小沼). At the southern edge of the strip lies the small eponymous village and tourist center of Onuma.
The nearest major center is Hakodate.
Leaving Hakodate, if you take the Ltd Express Super Hokuto train, it takes 19 minutes to reach Onuma Koen station. The regular JR Hakodtate train takes about 50 minutes. The trains leave about once an hour for the park. Be sure to check the return schedule because trains are more infrequent in the evening and they don't run very late so you may find yourself missing the last one.
There are also buses to Hakodate that take a comparatively slow 60 min for the journey.
A less frequently traveled branch line from Ōnuma station runs along the southern shore of the lake.
There are a few easy-to-spot places that rent bicycles to tourists. You can get normal bicycles, bicycles built for two, and tricycles that can interlock with each other (for families and groups).
The lake is known for its 126 islands.
Hiking in the area is popular.
There are many hot springs scattered around the shores of the lake.
Although you can buy food from little shops and stalls, you might consider bringing your own food in case you find the local tastes disagreeable.
Onuma Beer is a popular local brew sold at most stores in the town.