OK - if you want to get there quick, you're going to drive through some of the least interesting stuff this country has to offer (Iowa, Nebraska). But if you want to do it right...
1) When you get to Chicago, stay on I90 West which cuts northwest so that you can get to South Dakota to see Mount Rushmore & Badlands National Park
2) In Wyoming get off the 90 on 16/20 to Yellowstone. Then down 89 to Grand Teton National Park
3) In Idaho, Craters of the Moon is semi-cool
4) South to Reno, NV
5) on to Yosemite. Kings Canyon National Park is also cool, as is Sequoia National Park
On the way back, take a more southern route, through Colorado. Rocky Mountain National Park, Dinosaur National Monument, Denver/Boulder, KC, St. Louis, Indy.
Yeah, I've done this.