I've been over all of this ground many many times, and listed out some of my favorite stops here.

I'll add to that, if you are semi-typical 21 year old college students, you'd probably enjoy spending a little time in LaCrosse, WI, which has the World's Largest 6 Pack (actually filled with beer), a free Brewery tours with samples, and far more bars than any other town with 50,000 people!

There's also a lot more to the Black Hills than just Mt. Rushmore and Crazy Horse.

You might want to consider stops at Wind or Jewel Caves, the Minuteman Missile Site, and Deadwood, just to name a few places in the area.