I did a trail ride down into the crater on Haleakala a few years ago, and much to my wife's chagrin, was just as interested in taking pictures of that site than of the trail ride itself. Apparently the site is also shared with some University of Hawaii-managed telescopes, and is occasionally open to the public. (though likely not the Air Force area.)
There are all kinds of really interesting/weird military installations on the islands, though. My favorite might be WWVH.