Next time I am in the states I want to see:

• La Brea Tar Pits in Los Angeles to see the dusty old bones from the ice age.
• Ancient Bristlecone Pine Forest in the White Mountains of California, home to the oldest known individual organism, the Methuselah Tree. Saw an episode of Nova on the Bristlecone Pine and have always wanted to see them for real.
• North Rim of the Grand Canyon in Arizona. I've done better than most people from Arizona having been there, but I've always wanted to go back to see the other side.

Stuff has to have been around for a long time before I want to see it :)