Hi, and Welcome to the Great American RoadTrip Forum.

I am assuming you are starting in Estes Park, which could make North Platte and Des Moines as nicely breaking up the journey. Depending of course on where in Chicago you will be heading, and assuming you want to only take the interstates. There are however, numerous routes you could take. Get out a good paper map, and see just which takes your fancy. There are routes through the Sandhills areas of Nebraska and scenic routes through IA and IL. With the time available, you might find it beneficial as well as interesting to take some of these lesser driven road.

Lifey