Michigan state park campgrounds are a little too crowded and disorganized for us. Still pretty, but the sites tend to be an "area" which means campers will sometimes be positioned sideways, backwards, etc. I highly recommend Platte River Campground at Sleeping Bear Dunes. You're extremely unlikely to get reservations on short notice, but they have quiet a few walk-up only sites. I would call them and see if they have specific days / times when your chances are best to get a site. Also, in the Petoskey area is the Petoskey KOA. Although typical crowded sites like at most KOA's, this particular location is spotless, has all kinds of nice amenities, and is in a nice location.