Love the Wisconsin state parks, BUT they are very popular, and they take reservations up to 11 months in advance, so you may find somewhat slim pickings. If you want to work your way up to the UP, check out Harrington Beach SP, and Point Beach State Forest - the latter is one of our favorites.
If you want to stay in Wisconsin & explore the state, our most favorite SP of all is Wyalusing, near the sw corner of the state. Best sites are dry camping in Wisconsin Ridge CG. Others we liked, in no particular order: Willow RIver (near the Twin Cities), Bong (west of Kenosha), Perrot (another Miss. River park), High Cliff (on Lake Winnebago), Kettle Moraine (both north & south units), Council Grounds, Peninsula (one of THE most popular in the state), Potawatomi, and Yellowstone Lake (I couldn't resist). Kohler-Andrae, near Sheboygan, is nice too but is likely to be booked solid.
If the SPs are booked up, I'll post another vote for the Rustic Barn CG as well. Nice little place.