On my last week in Rome, I took a day and took the train to Ostia Antica, an ancient city outside of Rome. In its prime, it was the shipping port for Rome, being where the Tiber River meets the sea. Over the past 2000 years, the sediment has made it so Ostia Antica is now 4km away from the current seashore.
After having a nice relaxing lunch, I explored the rest of the day in this beautiful ancient city.
While not as complete as the ruins in Pompei, the ruins at Ostia Antica are lovely and in much better shape than most of the ruins in Ancient Rome. It is nice because walking around gives you a pretty clear view of how the masses lived in this time period, as many of the apartment complexes are still standing.