Most of Rome's monuments are lit up at night, so you can get a great deal of sightseeing done in the after-hours.
The walk I suggest begins at Piazza del Popolo (M: Flaminio), and continues down Via del Corso, veering off to see the Spanish Steps, the Trevi Fountain, the Pantheon, and Piazza Navona. At the end of Via del Corso is Piazza Venezia (watch out for traffic!!) and just across the piazza is the Vittorio Emanuele Monument, and just behind that on the left the Colosseum. The whole walk should take you under two hours. At this point, you can hop back on the metro at the Colosseum, or head down the street to Campo dei'Fiori and keep the evening going!