Yellowstone (Wyoming) is always fun; however, if you can stand the drive then Glacier National Park (Montana) is the best!!! There's lots of stuff to do up in that area of the world too (i.e. visit Little Big Horn battlefield, Devil's Tower, Badlands, Sturgis, Mt. Rushmore, etc.). If you're looking for something in the south, then try the Mesa's or the Grand Canyon. Have fun.