The Minuteman Missile National Historic Site is worth visiting. It's just a little east of Rapid City, South Dakota, near Badlands National Park. Not only do you get to go down into the control "capsule" with the blast door in the photo, you also go out to one of the nearby missile silos, complete with decommissioned Minuteman II missile.

I've got a whole set of photos of that visit on our family web site (pre-Flickr -- need to get the full-res versions uploaded there someday) here, including one where you can see the six- or eight-inch thickness of that door, and quite a few of the geek stuff down there. And then the missile visit, too.