Hop-On, Hop-Off Bus
Rome has multiple hop-on, hop-off sightseeing-bus tours, with competing operators aggressively trying to lure you with flyers, though City Sightseeing runs the ones you'll see most frequently. Note that Rome is actually not ideally served by bus tours, as most of the main sights are close together and on small streets not accessible by bus. You may find walking between sights easier than getting on and off buses constantly.
City Sightseeing. Hop-on, hop-off buses leave every 15–20 minutes daily, beginning at 9 am from Via Marsala (beside Termini station), on a 100-minute loop, which passes the Colosseum and St. Peter's, and makes stops close to the Trevi Fountain and Piazza Navona. www.city-sightseeing.com. From €25.