All I remember between Denver and Chicago is, well, nothing. It's vast and empty out there, not much of note to see or do. Even with 6 days you still need to cover about 600 miles/day, so there won't be a huge amount of time for sightseeing.

I would drop south earlier and avoid Chicago. It's a great city, but you lose too many hours tied up in traffic.

Had to go to Omaha once. It had a surprisingly funky and lively downtown scene in the old warehouse district. SRJ may be able to advise, I think he makes annual pilgrimages there.

What, no Texas? After all the abuse you give us it only seems fair that you come experience the place firsthand!