Heading to Denver from Illinois, suggested stops?

I made that trip many a time. Check out Starved Rock in Illinois on the Mississippi River. The Lincoln Zoo in Nebraska is worth a visit too. Cabela's in Kearney or Sydney are absolutely worth a visit. Lake McConaughy if you want to cast a line for some walleye and trout, or just take in the sights. If you go through Wyoming, spend a day or two in Vedauwoo among the granite boulders, pines, and beaver ponds. Great camping to be had there. Perfect time of year for your trip!