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.
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.
Holiday World in Dale, IN
it's every bit as good as six flags, only it's not overcrowded. Also, free drinks!
I enjoy the outdoors a lot. Hiking, just sight seeing is fun too. I always enjoy parks and just going off the trail(safely) where there's less people.
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/