My wife and I are volunteering at a camp in Montana this year. It is located about 5 hours from Yellowstone, so we figured we would take an extra week and do some site seeing. We will NOT be camping t...
I would eliminate Devil's tower if it buys you another day in Yellowstone. It's a long, boring drive to get out there and it is one place that a quick glance sort of does the trick.
Don't underestimate the Black Hills area. For us, it wasn't far behind Yellowstone.