Ostia Antica is the old harbour of the Romans, and visiting it is like visiting Tivoli and Hadrian's Villa: a relaxing day trip that takes you away from the noise and sirens of Rome. Reserve a whole day — not just to relax, but because Ostia deserves it.
The best way to get here from Rome is by metro. Trains leave from Piramide station: get off the regular metro at Piramide, go up the escalator, turn immediately left and down the steps into the Roma-Lido station. A normal metro ticket covers the entire journey. Get out at the stop Ostia Antica.
There is a bar on the site, but it is worth picking up food in the small, enchanting modern village of Ostia Antica, two minutes away. Turn right towards the medieval fortress to reach the village, where you will find a good alimentari that will make you a bread roll with ham or cheese. The best ice cream (made on the premises) is at "Il Gelataio" next door.
Many visitors make the mistake of spending too much time inspecting the first buildings they encounter. Consider starting by following the modern road to the right, which leads to the museum (closed in the afternoon). The bookshop here has guides to the excavations — the best one, by Carlo Pavolini, is unfortunately only available in Italian. In front of the museum, under the shade of a pergola, you can plan your route.
A word of caution: it is not a good idea to pick up stones with bare hands, given the presence of what the archaeologists fondly call "our little friends" — black scorpions. Not deadly, but best avoided.