Chicago to Glacier NP during Stugis

Theodore Roosevelt National Park comes to mind as one possibility. Pretty (badland) scenery, wild horses, prarie dogs, and buffalo are the main attractions. It's not in my opinion one of the top tier of our National Parks, but that doesn't mean it's bad.

If you cut up to US 2 somewhere, which I imagine you'll probably do, I rather enjoyed the Havre Beneath the Streets tour in Havre, MT. I think it's especially geared towards the younger (school-age) crowd, but it's fascinating regardless.

In between the two I've camped at the Nelson Lake BRC campground, which has very limited ammenities, but is pleasant lakefront camping and the price (gratis) cannot be beat.

Edit: Havre also has a very well-preserved buffalo jump site that I've heard is excellent, though I didn't see it myself.

Jamestown has a HUGE buffalo statue, and a rather interesting little museum. Farther west, Theodore Roosevelt NP is worth a day or two, IMO.