Tivoli is a relatively short train ride from Rome and has been a popular destination for excursions and a countryside residence for Romans since ancient times. While the Villa Adriana is a few kilometres outside the centre, Villa d'Este is right in the centre. Commissioned by a Renaissance cardinal, it is considered to be the defining masterpiece of Italian landscape architecture and garden design. The garden is indeed very pleasant for a stroll, full of fountains, ponds and other water features, although I visited a bit too early (in early March) to enjoy the full spring bloom. The villa itself, though, was not particularly impressive, and can be skipped if you are short on time. Besides the 2 villas inscribed on the WH list, Tivoli features a third one that is in my opinion the equal of the other 2 - the Villa Gregoriana, famous for its unique combination of a natural landscape and one altered by men since antiquity. I certainly enjoyed my walk down to the valley floor and up again on the other side, always admiring viewpoints, caves, waterfalls, and a number of manmade structures. It has been on Italy's tentative list for a long time, but I doubt that UNESCO would inscribe a third separate site in such a small town - an extension of Villa d'Este would be justified, though.