I've never been to Mackinac Island. I think I'd fly into Mackinac County rather than directly onto the island, mostly because it will be a lot cheaper to get a room. The ferry over to the island runs every 40 minutes or so, and costs about the same as the landing fees at the island airport. There's no av fuel on the island, either, and I'll definitely need gas. There's a water front Days Inn just about a mile from the county airport, so that's most likely where I'd stay.
I think this is an even better idea: fly into Sault Ste Marie, Michigan (KANJ) and do the Agawa Canyon Tour Train. Yeah! I'm going for the Premium Dome Car Service, too. There are plenty of motels near the airport and there are supposedly rental cars available. I think this would be awesome in the fall!
I think I'll stay here.