Went through some of that same territory on my cross country trip last fall. Some things we saw along the way:
Wisconsin Dells (just drove through)
Spam Museum (MN)
Blue Earth, MN
Corn Palace (Mitchell, SD)
Badlands, SD
Wall Drug (Wall, SD)
Custer State Park, SD
Jewel Cave, SD
Wind Cave, SD
Mt. Rushmore (but, that's already on your list)
Crazy Horse Memorial, SD
These spots in Wyoming are probably out of your way, but...
Devils Tower, WY
Bighorn Mountains
Bighorn Canyon, WY (more specifically, the Pryor Mountain Horse Range)
Yellowstone
Grand Teton
Jackson Hole
Have fun on your adventure!