Ostia Antica is massive, far bigger than I expected; you could wander here for hours. Bring the free Rick Steves audio tour on your iPod and a picnic lunch. Look around for a while, find a quiet spot to lay out your spread, relax, then look around some more.
There are parts of this place that are unexcavated, where you can lie on a peaceful grassy field and look over ancient ruins 50 yards away. It's pretty amazing.
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