Cross country ski trails and snow shoe trails.
There's a ton of stuff to do in winter: Trail hikes, night time bird watching, a marathon, etc., Check the gate house when you enter the park. There's a list of upcoming events.
In winters, nothing you can do there
Just a warning, the shelter restrooms are closed in the winter. North and South entrance porta potties are always there.
Besides the beach area most things are still open.
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