Pick up some fresh sweet cherries at one of the roadside stands. They're grown around Traverse City and Lupton. If in Mackinaw City, get some homestyle cinnamon bread and pecan rolls at the little bakery a block north of the dock entrance. There are several nice lighthouses to see around where you'll be... one in Mackinaw City, another just west of town (2 or 3 miles maybe?), and a couple more toward Rogers City.
Take jackets; it can get cool along the big lakes, especially at night. Mackinac Island is typically about 10 degrees cooler than the mainland. I pack a warm coat whenever I go to MI, and sometimes I've been glad I did.