How can you go to Vegas and not go see the Hoover Dam. Getting there and back isn't fun but like everyone says it's just one of those places you have to see, even it's just once.
I didn't do the tour, just parked ($7) and walked around to check it out.
Also, if you have time, check out the Hoover Dame Store, their slogan is that they have "the best dam gifts in the west". The people who worked there were hilarious or at least they amused me. They try to throw dam into every sentence and then when you're leaving they say "have a dam fine day". That stuff kills me.