Mackinac Island is great. Don't miss items include a carriage ride, the butterfly houses, and if you like history Fort Mackinac. The Grand Hotel also has a walking tour that you can take.
Also, if you have a lot of time on your trip, don't limit yourself to Mackinac, there are a lot of interesting things in that area to see. Across the bridge in Mackinaw City is shopping, Colonial Michilimackinac, and a boat museum. Traverse City is also not that far of a drive. In the Upper Penninsula, it's about an hour to Oswalds Bear Ranch (a great place to interact with black bears), Tahquahmenon Falls, Lake Superior. St. Ignace also has some nice shopping, Castle Rock, and various 'touristy' attractions that can be fun. You can find info on all of these items online, both at their own websites and at michigan.org. Have a great trip!