I've always found that the US states of Nebraska and Kansas are best seen either in excess of 100mph or at around 10mph. I also enjoyed riding Alberta/Saskatchewan/Manitoba on trip across Canada, etc.

There is a surprising amount of variety still when you go at 10mph. You aren't on the largest highways (which I do find monotonous - hence the 100mph comment) but instead you'll see and notice more subtle changes (e.g. huge change in KS as you cross the 100th meridian and elevation also drops) and more subtle things (e.g. kernels of wheat trapped in cracks of the road during harvest) and come through some interesting small towns or even remnants of towns. An extra bonus if you come through some of these towns on special days (e.g. fun memories of 4th of July in Peabody, KS or Delta, UT).