Arecibo Observatory

The nerd highlight of my trip to Puerto Rico was visiting the world's largest radio telescope dish, at Arecibo Observatory, technically the National Astronomy and Ionosphere Center, run by Cornell for NSF. They took a whole valley, rounded it out a bit, and covered it in aluminum mesh.
It's big. Really big. So big you need an aerial view to really see the whole thing. (1000 feet across, with the transponder 500 feet above the dish.)
Plus, they move the transponder in the middle every half hour or so! I had a blast.

The dish has appeared in two major movies, Goldeneye, in which it played a secret Russian radio transmitter in Cuba, and Contact, in which it played itself.

My best souvenir was a bank machine receipt that says "ATM: Radio Telescopio" on it.