The Hoover Dam - http://www.usbr.gov/lc/hooverdam/
If you like incredible feats of engineering, or just wanted to know what keeps all the lights on in Vegas, the Hoover Dam is about an hour’s drive from the strip and has daily tours. It is a pretty incredible site to see in real life. They have hourly tours ranging in price from $20 to $30 depending on how much of the dam you’d like to see, if you go on any tour, I recommend the $30 tour as you get to see far more of the insides of the dam. The $30 tour requires about 90 minutes, a decent amount of walking and you should arrive about two hours before you want to be on it because the waiting list builds up quickly; I went on a Thursday morning, arrived at 10:30 and the first open slot was just after noon. So wear comfortable shoes and don't go if you're not okay with a lot of walking/steps/small confined spaces (you're inside a dam after all).