This property of the Rockefeller Foundation has celebrated gardens on the site of Pliny the Elder's villa, overlooking Bellagio. There are only two 1½-hour-long guided visits per day, restricted to 30 people each, and in May these tend to be commandeered by group bookings. It's wise to arrive early to sign up. It also closes due to bad weather, so call in advance.
Near Palazza della Chiesa, Bellagio, 22021, Italy