If you take the middle route -
Consider Route 30 which runs from Atlantic City to Oregon.
Known as the Lincoln Highway, I personally know it from the Indiana/Ohio State Line West into Idaho. Two lanes (mostly) as you get to Wyoming it merges with I-80 for a bit.
You'll see real small town America at a nice pace.
And it gets you to Wyoming, Yellowstone, and rides like the the Beartooth Pass quickly.:usa