Hoover Dam/Lake Mead, Las Vegas
If you're going to go to the Hoover Dam, rent a car, don't go on one of the tour buses. They are a pain, take forever. If you go in the summer, it's hot, bring lots of water. When we went in 2003 the resevoir was so low that water was not flowing over the dam, which to me was going to be the highlight of the trip right! So we just got to stare at this big dry block of cement dam wall. Dam!
The Hoover Dam is interesting in all respects, how it was built, but kind of a long boring trip, better to watch a show about it on the History Channel.
Fun Alternatives: Is there an alternative? The Grand Canyon would probably be better.
One of the greatest engineering marvels of the 20th century is only about 30 mins. outside of Las Vegas and definitely worth a look. It's not as cheap as it used to be but they built a monumental parking garage and visitor center with really swank views to the dam. It's sort of too hot to visit in summer in my opinion but it's worth the trip, plus you get to go through the cute city of Boulder City on the way which was built to originally house the dam builders and their dam families.
Unique Suggestions: Take the tour which takes you actually inside the dam where you get to see the gigantic turbines and penstocks (water tubes). Take note of the swanky art deco statues and details which make the dam so beautiful. There is also a really cool view from the restrooms at the top if you stick your head out the tiny little window (it looks straight down the face of the dam).
Fun Alternatives: If the dam tour is really that terrible you can always kill yourself afterward. Dozens of people have said, 'adios cruel world' and jumped from the dam (over the non-water side of course, which is kind of gory considering you would bounce once and slide all the way down to the bottom where you finally splat).
One particularly expensive tourist trap is the Hoover Dam. If you can catch a tour it's worth it, and the parking is free, but you better win a jackpot to be able to afford a hotdog, a roll of film, or any kind of souvenir.
Unique Suggestions: Take the tour, pack a lunch, bring extra film.