Drive US 2 between St. Ignace and Engadine for some nice views of Lake Michigan.
In Mackinaw City, if you like good bread be sure to stop at the bakery a block north of the main shopping area; I always try to get a loaf of the cinnamon bread. Of course, there's plenty of fudge, caramel corn, and fish to eat around there, too.
My favorite thing to do on Mackinac Island is to ride a bike all the way around the outer perimeter, enjoying the lake views.
If you go west in the U.P., I'd suggest Palms Book SP where you can float across the Big Spring... perfectly clear water with fish 40' below that look like they're only 10' away. Also check out Fayette SP and the historic buildings and artifacts there. As previously mentioned, the Pictured Rocks Lakeshore is worth seeing by boat (pricey, but worth it... try to book the sunset cruise on a sunny day for best light on the rock cliffs). Tahquamenon Falls is worth visiting, as well as Whitefish Point's lighthouse/tiny museum.