Although its pronounced “mackinaw” its spelled Mackinac.
On the island take a bicycle ride around the island. Hire a carriage for an island tour.
Walk up and take a look at the Grand Hotel (costs $10 just to walk on the porch!).
Go up the hill from the main street to the fort. You can get your fill of T shirt shops and fudge shops on the main street (the tourists are called “ fudgies”). A walk through the municipal marina can be entertaining, there are sometimes very interesting boats there other than the typical powerboats.
If you’re there after the daytime tourists take the ferry back to the mainland, the island becomes a completely different place.
If you get off the island for any length of time! the northwest corner of the lower peninsula is interesting with places like Petoskey, Harbor Springs, and Charlevoix.