If you are starting in Illinois, I am assuming you are either taking interstates 70 or 80 to 29 and then hit 90 in Sioux Falls. If so, be sure to go into Sioux Falls and see the park around the falls the city is named for. It is a really pretty area. When we were there, the lady at the SD Welcome Center told us about it, and it was a really nice little 15 minute stop.

While you are at Little Bighorn, make sure you go out to the Reno-Benteen Battlefield. There is a road that follows the ridge, or you can (I believe) walk along the river where the Cheyenne and Lakota were camped.

Wall is somewhat of a tourist trap, but it is still interesting to see. Apparently it started out as a drugstore that offered free water to travelers (and they still will give you a free cup), but it has grown to a huge trading post with restaurant and all sorts of things - a boot outlet, clothing, souvenirs, etc.

Mount Rushmore shouldn't take very long. It is impressive, but once you see it you've seen it.

Very jealous of going to Glacier. When we were out there it was late May and the park was still pretty much closed due to snow, so we didn't make the trip up from Missoula.

As someone else said, if you are at Yellowstone, make the trip down to Grand Teton. It is a beautiful drive, and the Tetons are an amazing range. One of my best pictures from that trip is looking across Jackson Lake to Ranger Peak.

Yellowstone, just take your time and get out on some of the trails. You can do the double loop in a day if you wanted, and you will see some cool stuff, but take the time to get out and see some of the less travelled areas and you will really be in for a treat.