Originally Posted by
Radux
If you're heading north from the Grand Canyon (slightly east from where you're going to be at), I'd definitely recommend heading up to Yellowstone National Park and see Old Faithful (and some other cool stuff). Since you're in that area (Montana), take the Going to the Sun Road to Glacier National Park for more awesome, though similar, stuff -- about a 7 hour drive total for some great memories if you're into the outdoors.
If you're more into the whole city thing and can fly out east, New York City + Time Square could be cool. Maybe check out a broadway show, too.