Villa Adriana at Tivoli is definitely a place to go if you want to escape from Rome for the day, or even a half a day. Located just east of Rome, Tivoli is not a difficult place to reach. There’s also the Villa d’Este, a beautiful Renaissance garden that is also worth a stroll. Villa Adriana and the archaeological park that surrounds it is pretty enormous, so you’ll need at least a half day there. The stroll through the park is quite pleasant, and there’s also a wonderful museum that contains many artifacts that were recovered on the grounds. Entrance to the museum is free and is included in your ticket.
Villa Adriana is also a UNESCO World Heritage site.
UPDATE: Tourist information (ticket prices, hours of operation, etc.) web site wasn’t working when I checked today, 4/22/2011. This site has some useful information (in Italian). I’ll work on getting some decent information in English up soon.