The Vatican Railway Station
Originally uploaded by gashwin
This isn't that well known, and it's relatively easy to apply for tickets. And it's definitely worth it, especially in the kind of great weather that we got a couple of Saturdays ago. The tour lasts about 2 hours, and one gets to wander around large chunks of Vatican City, ordinarily closed off to the public.
Photos are up at Flickr, in their own set.