The ruins of Hadrian's vast country villa, 5km outside Tivoli proper, are quite magnificent, easily on a par with anything you'll see in Rome. Built between AD 118 and 138, the villa was one of the largest in the ancient world, encompassing more than 120 hectares – of which about 40 are now open to the public. You'll need several hours to explore it.
Must-see's include the canopo, a landscaped canal overlooked by a nymphaeum (shrine to the water nymph), and the Teatro Marittimo, Hadrian's personal refuge.
Hadrian, a great traveller and enthusiastic architect, designed much of the villa himself, basing his ideas on buildings he'd seen around the world. The pecile, the large pool area near the walls, is a reproduction of a building in Athens. Similarly, the canopo is a copy of a sanctuary in the Egyptian town of Canopus, with a narrow 120m-long pool flanked by sculptural figures. At its head, the Serapaeum is a semi-circular nymphaeum that was used to host summer banquets. Flanking the water, the antiquarium is used to stage temporary exhibitions (note that when these are on, admission to the Villa costs slightly more than usual).
To the northeast of the pecile, the Teatro Marittimo is one of the villa's signature buildings, a mini-villa built on an island in an artificial pool. Originally accessible only by swing bridges, it's currently off-limits due to ongoing restoration.
To the east, Piazza d'Oro makes for a memorable picture, particularly in spring when its grassy centre is cloaked in wild yellow flowers.
There are also several bath complexes, temples and barracks.
Parking (€3) is available at the site.