About 20 miles outside of Rome is 400 acres of the ruins of an ancient Roman harbor city, called Ostia.
Ostia was founded around 620 BC, and rivals Pompeii, yet is only 30 minutes from Rome by the metro train. I took the train out and spent a couple of hours one afternoon exploring the ruins. Unlike most of the ruins in Rome, you’re allowed to wander in and around most of these. There’s the bath house, arena, shopping arcades. Some of the ruins even still have the ceilings on them, so you are able to walk into the rooms and imagine what it must have been like 2000 years ago.
If you visit Rome, try to make time to grab a train to Ostia Antica. It’s well worth the trip.