The Grooming Report lists which of the ski trails bikes are allowed on. You need a trail pass for that. There are also groomed bike trails around Pearygin Lake which are free. It's really hard work! As soon as you leave the trail, the front of the bike sinks deep into the snow. But as long as you stay on the trail, it's fine. As a bonus, the fat bikes can handle snowy, icy roads, too.

Before my first winter there, I saw this photo and got excited. If you google "fat bike Winthrop WA" it's one of the first that come up. Ironically, it's a picture of Rainy Lake where bikes are illegal.