Hoover Dam

In the 1930's, the mighty Colorado River was damned up to provide a stable water supply for farmers, and electricity to Arizona and Nevada. It was quite an engineering feat - the river had to be diverted into tunnels during the construction, and the concrete had to be built in interlocking blocks (the ultimate Lego project). In 1935, when Hoover Dam was completed, it was the world's highest dam (726 ft high).

Visitor Center - Stop into the visitor center for hands-on exhibits, models and dioramas of the dam construction, and a short introductory movie.
Observation deck - From the observation deck are stunning views of Lake Mead, the Colorado River, and Hoover Dam.
Powerplant Tour - From the visitor center, take an elevator 500+ ft down for a 45 minute guided tour. On the tour you'll see the massive generators that convert the water flowing through the dam into electricity, and go through tunnels deep in the heart of the dam.
Hoover Dam Tour - The Hoover Dam tour includes the Visitor Center, the Powerplant, and an additional section, the inspection tunnels. The tour is for kids 8 and up, takes 1+ hours, and is fair amount of walking.
Walk along the top of the dam - Walk along the sidewalks on the top of the dam over to the intake towers, for stunning views of the lake. On one side of the lake is Nevada, on the other is Arizona.