We've all heard about and seen pictures of Hoover Dam, and it certainly is impressive to see it in person. The problem with visiting is that there are plenty of Americans also waiting to see this engineering marvel, and it seems like a large majority will arrive on the same day you do. Crowds, even in the early spring, are too large even for the newly-constructed visitors center.
There are several different tours, and the basic tour will take you to all the impressive parts of the dam. A more expensive, and longer, hard-hat tour is available for the more adventurous. You will probably stand in line for upwards of an hour if you go while the dam is moderately busy.
A visit to Hoover can be memorable, but it requires a good deal of patience any time except the off-season (winter).
Image by United States Department of the Interior