Hoover Dam. This is an easy one. If you're on a family vacation, rent a car for a day, and drive out to the Hoover Dam for this once in a lifetime experience. Pay for the extended tour, it's fantastic! We went in Feb so the temperatures were perfect. I, and the staff, suggested you do not visit on extremely hot days as the inside temperatures will exceed 130 degrees in the tunnels which you'll be walking in. Go! You will love it.