That's still under 10-12 days, so another option is to change your route. I would personally go through South Dakota and spend time in the Badlands National Park, Black Hills National Forest, Custer State Park, and of course Mount Rushmore if you haven't done it recently. Or alternatively, go from Chicago to Winnipeg and spend more time in Canada. Either way, having more days gives you the luxury of exploring each stop at a relaxed pace. Have a great trip!