Just the UP or all of Michigan?
The entire West coast of lower Michigan is beautiful ... Grand Haven state park is _VERY_ popular, likely it's already sold out for this year. I don't understand it myself, not my idea of a "park".
Further North, there is the Sleeping Bear Dunes National Lakeshore. The campground is a privately managed federal park, kinda pricey IMHO (park fee, camp fee, shower fee, etc) but the scenery is worth seeing. The dunes themselves are absolutely humongous piles of sand, some folks enjoy climbing.
Southeast Michigan has "The Henry Ford" and Greenfield Village - if you take I-75 to Mackinaw you'll be pretty close to these destinations. 15 years ago, Oakland County (North of Detroit along i-75) boasted some of highest-end shopping (and property values) you could find outside of Hollywood and New York, not sure that's still the case.
