Fish boils in Door County (very different than a fish fry). Check out the Great River Road along the Mississippi, southwestern edge of the state. Our favorite Wisconsin SP is there (Wyalusing).
Yes, it's kitschy and touristy but you will never see anything else like the House on the Rock. Even my parents, who HATED kitschy places, loved it. Agreed, Mirror Lake SP is the best CG in the Baraboo/Dells area, but I'll put in a plug for the Wis. Dells KOA (since friends own it. :) ) We enjoyed the Duck rides and the boat rides in the Dells, and if you have an interest in water parks, well, that's water park heaven.
Also agreed that the SPs along Lake Michigan are gems. (And very hard to get into.) Point Beach State Forest's campground is another favorite of ours.
Longdrive, believe it or not, we lived 32 years along the IL/WI border and never ever stopped at Mars Cheese Castle. I kinda wanted to, though. :)