The Leelanau Peninsula is one of the most beautiful places in Michigan. Sleeping Bear Dunes also fabulous.
If they come as far as the UP, Mackinac Island is a lovely place but expensive, and touristy downtown. Ferrys run from both Mackinaw City and St. Ignace.
Sault ste. Marie has boat tours that you can ride up and down through the locks, and an area where you can view ships going through the locks. There is also a museum ore freighter called the Valley Camp which is a fun tour.
Just west of town is the Iroquois point lighthouse, with a fabulous stony beach.
Farther west is Paradise, with a shipwreck museum, and Tahquamenon Falls.
Pictured Rocks is beautiful, and there are tour boats and kayak tours out of Munising.
Just west of St. Ignace along US 2 is several miles of Lake Michigan sandy beach stretched alongside the highway.