Day Trip to Castel Gandolfo & Lake Albano is a real time commitment — fit it in only if it's a priority.
Allow Half-day / full day.
A pretty hill town over a volcanic-crater lake in the Castelli Romani — but the genuinely new reason to come is that the Apostolic Palace, the former papal summer residence, plus the vast Barberini Gardens are now open to the public for the first time in centuries. You can tour the popes' private apartments and ride a little electric train through the gardens. The town itself is relaxing rather than spectacular, so it's the palace-and-gardens opening that lifts this above a generic lakeside stroll.