Trip Ideas
If you are going to Yellowstone (one of the nicest places I've ever been) you can go a little farther and make it to Glacier National Park at the Montana Canadian border. You can also visit South Dakota and see Crazy Horse, Mount Rushmore, The Bad Lands, Devils Tower (its the place where Close Encounters of the Third Kind takes place).
I'll be heading to Glacier National Park in the middle of May.