Take a light coat or sweatshirt with you. That island can get cool with a lake breeze. The island is basically a tourist trap, but has some historical significance. Ft. Mackinac was occupied by the British, the US, and the french. The downtown area has those little shops that women seem to love, but also has a few bars to grab a cold one too, if you know what I mean.
You'll exit east off I-75 before you get to Mackinaw Bridge. Go thru the downtown area and there will be the ferry services not far away. Just look for signs for Sheppler's or Arnold's ferry boats.
And bring your wallet. That island isn't cheap.