You'll drive right by/through Yellowstone and Grand Teton National parks. My favorite places on earth. You could spend a week between the 2 and just skim the surface.

Also, if you go through either or both parks, you need a national parks pass of some sort. An adult pass is $80 for nationwide access for a year, I forget what the single entry fee was for both parks -- something like $25 US for a week. Seniors get a lifetime pass for $10, but not sure if that extends to non citizens.