For centuries a closed world, the papal gardens at Castel Gandolfo can now be visited, albeit accompanied and in groups. The regular one-hour tour involves a mini-train ride through the extensive gardens, taking in Roman ruins, flower displays, woods, fruit-and-veg patches and the papal helipad.
To explore on foot, there are scheduled Saturday morning visits when you're free to walk around accompanied by Vatican staff – see the website for details of this and other visitor packages.