May will be cool (figure highs in the 50-60 range depending on where you are)
The lakes will be too cold for swimming. The west coast has incredible beaches and dunes with nice tourist oriented towns.
Tahquamenon falls and Pictured Rocks in the UP is impressive and hard to leave out Mackinaw.
Honestly, Frankenmuth is overrated.
If you get down near the Detroit area, consider a day at the Henry Ford Museum and Greenfield Village and if you like museums the Detroit Institute of Arts is a really great museum.