The Buffalo Bill scenic highway, the drive west from Cody to Yellowstone, was fantastic! I think it was Teddy Roosevelt that said this was the most scenic 50 mile drive in America. I don't know that I'd put it number one, but it's up there.
The Visitor center video, showing bison attacking people and elk ramming vehicles, is a great place to start. It should keep some of the overly ambitious in check.
Beautifully icy Lake Yellowstone
~8000 feet above sea level at Yellowstone, driving through the park you cross the continental divide several times.
Old Faithful – Entering the area looks like the entrance to Disney World (they are setup for mass numbers of people), but it's still pre-season so it wasn't too crowded.
The water spouting out started seeping in the ground hundreds of years ago – maybe that's what puts the “old” in Old Faithful?