The Island is GREAT but remember you can't drive there you have to take a ferry.
Several years ago we camped at "Mackinaw Mill Creek CG" about 3 miles south east of Mackinaw City on US 23. Their facilities were very good and a current check of rates shows that they are still reasonable. Also I suggest that you visit "Old Mill Creek SP" just down the road. It has an operating water powered vertical saw mill, pit saw and a zip line.
I assume that you're going south in "Upper Michigan then through Wisconsin. If so stop for a "fish boil" in Wisconsin. It's real good eating with white fish and potatoes.
Dave