I also recommend Ostia Antica. I think it was an easy 1 hour bus ride from downtown. It is a full port city, 1 mile long. You get a map and full reign to wander

I also recommend Ostia Antica. I think it was an easy 1 hour bus ride from downtown. It is a full port city, 1 mile long. You get a map and full reign to wander around it and explore the ruins, including the very cool public bath mosaic. Most of the time, we were the only ones in eyesight, so you really feel like you have the entire ruins to yourself.