If your trip to Castel Gandolfo is all booked but your travel itinerary is looking a bit empty, let us give you a few hints and tips.Plan a stop at Papal Palace of Castel Gandolfo. It's one sight you'll usually find on travelers' must-do lists. If you still have room on your itinerary, Palazzo Colonna and Palazzo Chigli will keep you amused as well.