I've been to (and stood on the edge of) The Hoover Dam, and it looked mightily impressive. However, l'd liked to have been there recently (alright, a few years ago!), because there had been a severe drought. Because of this, the waters had been receding, and exposed the old Mormon pioneer town of Saint Thomas, which was covered over by the Colorado river when the damn was built.
Now that would be an interesting thing to see, and maybe the last opportunity to do so.
The last two photo collages are from here.