Best thing to see between Denver and Chicago?

My friend and I are going on a roadtrip from vegas to connecticut, and it seems like there is nothing to see between denver and chicago. I’ve been through kansas already- any alternate routes that take us by any good landmarks, state parks, national parks, anything?

The most interesting place out that way (although it’s not directly in line) is St. Louis. There’s the Mississipi, with it’s paddle-steamer casinos, lots of historical sites – it’s where the Lewis-Clarke expedition started from, and was considered the edge of civilization in Wild West times. The most striking feature of the city is the Gateway Arch, on of the most amazing man-made structures on Earth; I’d happily go back there just to see that again.