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 Hakodate 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.
- The most popular courses through the little islands that dot the southern shore start from near Onuma Koen station, and the time-pressed can even opt for a 15-minute circuit. Ask at the tourist office for a map.
- Paddle boats and row boats are available to rent.
- Mt. Komagatake (駒ヶ岳). This volcano dominates the scenery, but the mountain has been off-limits to climbers recently.
There are many hot springs scattered around the shores of the lake.
- Yūtopia Onuma (ユートピア大沼). Tu-Su 10AM-9PM. A modern hot spring facility whose main feature is its convenient location near the campground. ¥370.
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.
- Higashi-Onuma Yaeijō (東大沼野営所). Wonderful little lakeside campground at the eastern tip of the lake, nearest train station Chōshiguchi. Toilets, running water, communal fireplaces all provided for free and the Yūtopia spa is only a short walk away. Beware of thieving foxes!
- Drop by the tourist office in the grandly named Onuma International Exchange Plaza (大沼交際交流プラザ Onuma kokusai kōryū puraza), near JR Onuma Koen, for maps and information (some in English too).