I agree with Silver Lake Yogi Bear. We go there every year.
We just went to Hartwick Pines. The campground was very nice but I wouldn't go in the middle of summer. There's no pool and no beach anywhere close by.
Tawas Point State Park is very nice. You have both shallow calm bay and the open water of Lake Huron with big waves.
When we went to the UP we stayed at Brimley State Park and used it as a central location to go to Mackinac, Tahquamenanon Falls, and Pictured Rocks. If you're going to the UP, you have to go to Munising and take the boat tour of Pictured Rocks. It's beautiful.