Less than an hour from central Rome by public transport, the scavi (excavations) at Ostia Antica are perhaps second only to Pompeii in giving an impression of what a town was like in imperial Rome.
Take metro line B to EUR Magliana and then change to the overland train to Lido di Ostia, getting off at Ostia Antica. A five minute walk from the station takes you to the entrance to the site. You can buy a guide book at the ticket office but most areas in the site are well signed in Italian and English.
As you walk through the entrance you enter an area of tombs which were placed alongside the road from Rome to Ostia, just outside of the city walls. You then pass through the remains of the gates and into the town itself.
Ostia was the nearest port to ancient Rome (although the coastline has now receded), and many of the buildings in the town were related to this - in particular the Forum delle Corporazioni where the trading corporations in the town identified their offices by mosaics illustrating their function.
The Thermopolium was my favourite building - a small corner trattoria which sold hot food and drink. Built in the 3rd century it is still remarkably intact with wall paintings still visible, an amphora sunk in the floor, and a small courtyard to the rear where you can easily imagine customers enjoying their refreshment.
We spent around three hours wandering (admittedly slowly) through the town and saw about half or it. Some of the mosaics still remaining are quite stunning.There is a small museum, with a new gift shop and snack bar to the rear. I would thoroughly recommend this day trip to anyone visiting Rome for more than a day or two who wants to discover about life in ancient Rome (just bear in mind that the streets are uneven and may present problems for anyone with mobility difficulties).