I have been to Mackinac Island many times. You didn't mention whether you were staying on the island or which side of the bridge you were coming from. There are plenty of places at different price points to stay on the island but would recommend reservations. The same goes for Mackinaw City or St. Ignace. My cousins own a resort, Pines of Paradise, which is just south of the bridge that has nice cabins on a lake. They have a website. If you're not from Michigan or the area here are some tips. If you are then my apology for telling you things you already know.
Here goes:
Even though it is spelled Mackinac the name of the island is pronounced Mack-i-naw (rhymes with paw)
Tourists are called Fudgies - you'll understand when you see all the fudge shops
People who live in the UP of Michigan (upper peninsula of Michigan) are called UPers - pronounced YOUpers because of the many Scandinavian residents who sound their o's as ou's
People who live in the Lower Peninsula according to the UPers are called Trolls - live beneath the bridge.
A staple of northern Michigan, especially the UP are pasties - pronounced PASTees. If you can get some locally try them.
There is a fort on the island that worth the visit from a historical point and the views from there are nice
There are no cars but the island is small so walking is not a problem. You can also take a carriage, rent a bike or a horse. You can take your own bike over on the ferry for a small fee if you want.
If you are not staying on the island don't miss the last ferry back to the mainlaind!
Hope you have a great time. It is a touristy but magical place.