Cruise to the island where you can do a short walk around it. Aboard the cruise ship, look out for deers grazing along the wateredge. They are held out of the main tourist area with fences but they will come all the way up to the fence for food.
There is a museum on local culture and wildlfe, some derelict Shinto shrines and great forests around it. There isn't much to occupy yourself with so most people hop onto the next cruise ship to return to the main jetty.
The night views around the onsen hotels are really pretty. You can snoop around the hotel grounds that are open to the public that are decorated with lights, the main street where lanterns are strung or walk around the rim as fireworks are released over the lakes (nightly between mid June to September)
Some hotels deliberately turn off the lights when the fireworks are being released so that guests can view the works from the dark comfort of their rooms
There are 2 main types of cruises. In the day, the cruise ship goes to two different islands where you can stop to explore the islands and pop back to the main onsen hotel area. At night, there are fireworks viewing cruises. The ship goes out to the middle of the lake where there is total darkness.
Motorboats that speed around the lake release the fireworks over the lake.
This is a beautiful curtain-walled restaurant with a great view of Lake Toya. The buffet dinner is excellent with both Western and Japanese cuisine.