Take a ferry over to Mackinac Island. This is a must see if you are in the area and have some extra time! The Island is a lovely place to hike and bike. There are horse-drawn carriages if you want a bougie experience. Plan on spending at least 6 hours.
You absolutely cannot be in Mackinaw without getting fudge! The top two are Joann's and Murdick's. Try a slice from both and let us know which one you like better!
There are countless parks that can give you an excellent view of the Big Mac! This bridge is the longest suspension bridge in the western hemisphere. The bridge is especially beautiful at night when it is covered in lights.
If you are extending your trip, it is worth checking out the following cities or areas: Charlevoix, Traverse City, Petoskey, Harbor Springs, or Sleeping Bear Dunes. The region has plenty of winery tours. If you are willing to venture up to the U.P., take a hike at Tahquamenon Falls or be mesmerized by the boats at the Soo Locks (That's where Morgan's brother, Blake, will be).