Walk Through Hoover Dam
Boulder City, Nevada
According to its tour guides, Hoover dam was built to last 2,000 years -- so there's no rush to catch a tour of its innards. It's fun, though... unless you don't like heights, or the thought that you're encased within umpty-billion tons of cement. If those are your personal fears, stay away.
Visitors take an elevator 54 stories straight down. Marvel at the giant spinning generators and immense pipes, then walk through a claustrophobic tunnel to catch dizzying views into Black Canyon while air currents blow into your face. All that neon in Las Vegas? It wouldn't glow without Hoover Dam.