Hot DAM!
How can you NOT love the Hoover Dam?! I always take out of town visitors to see the awe inspiring dam. Parking on the Nevada side is $7, but you can park for free on the Arizona side (which will be full/busy on weekends and holidays). I have never taken a tour down into the dam, but the place is impressive nonetheless.
I can not imagine the hell that the workers had to go through building this thing! It is truly astonishing.
If you have never seen this amazing structure, it is certainly worth renting a car and taking a day trip out through Boulder City.