Even on a tight schedule, Day Trip to Tivoli (Villa d'Este & Hadrian's Villa) earns the hours.
Allow Full day.
Two UNESCO sites in one outing: Villa d'Este's terraced gardens are a riot of hundreds of fountains, and Hadrian's Villa is a sprawling imperial estate to roam. They're a few km apart and reaching both by public transport is fiddly, so a tour or car makes the day far smoother.