Pictured Rocks National Lakeshore

House on the Rock in Wisconsin. It's pretty cool.

Anywhere along the shores of Lake Superior is nice, lots of good hiking

A few off the top of my head:

If you like theme parks and especially if you have kids, Holiday World in Santa Claus, Indiana is one of the best.

If you like hiking and the outdoors, Garden of the Gods in Illinois.

If you're into military history and/or stuff that flies, the National Museum of the US Air Force in Dayton, Ohio.

Driftless area: NE Iowa, SE Minnesota and SW Wisconsin

Cedar Point Amusement park in Sandusky, Ohio. There's not much else in Sandusky, but the park itself is world-class, people fly in from all over for the coasters.

The Minnesota Marine Art Museum, a hidden jewel in Winona, MN.

What kind of stuff do you like?

Holiday World in Santa Claus, IN

it's every bit as good as six flags, only it's not overcrowded. Also, free drinks!

The Fenelon elevator in Dubuque is pretty neat. I can't say it's as exciting as a roller coaster, but the views of the Mississippi are incredible. https://www.fenelonplaceelevator.com/

The National Mississippi River Museum and Aquarium in Dubuque is interesting too. They have old river boats there in the summer that you can tour, and the inside exhibits on the geology and wildlife of the Upper Mississippi are incredible. Really great interactive exhibits for kids, but I also enjoyed it as an adult. https://www.rivermuseum.com/