Area 51

Military bases aren't always tourist attractions, but the so-called Area 51 Air Force facility in Nevada is – mostly because it's top secret.
Historically, the facility known commonly (though not officially) as Area 51 was involved with building and testing experimental and classified aircraft and weapons. Today, what goes on at Area 51 is also classified, but the site is at the epicenter of lots of conspiracy theories.
Many believe that UFOs and the aliens piloting them have been captured by the government and brought to Area 51 for research. This includes items thought to have been found in Roswell, New Mexico. Whatever you believe, an entire tourist industry focused on extraterrestrial life has grown up in the area, including the small town of Rachel, nearest the site.

Practical Info

While you obviously can't get onto Area 51 itself, you can visit the Extraterrestrial Highway and the UFO-themed town of Rachel, and there are day tours that cover the area from Las Vegas.