Death Valley, CA
Crank up the AC! They don't call Death Valley "the hottest spot in the U.S." for nothing. In summer, the mercury routinely hits 120 sizzling degrees.
If you can stand the heat, you'll see the coolest stuff this side of the moon. There are rainbow-colored rocks (fuschia too!), ghost towns, funky desert museums, resorts, hikes, reptiles -- even real life road runners and coyotes.
Comfort is the name of the game. If possible, plan to come in early spring, when the scorching heat subsides (it only gets up to 85 degrees then). You'll appreciate this desert wonderland much more if you're not worrying about heat stroke.