Rome's crowds concentrate in about six places. Ten minutes' walk or forty minutes on a train gets you sites of comparable quality with almost nobody in them.
Ostia Antica
Rome's port city, forty minutes by metro and regional train.
It was abandoned gradually rather than buried suddenly, so it lacks Pompeii's frozen quality — but it has streets, warehouses, apartment blocks, a theatre and mosaic floors standing to considerable height.
Half a day, and you can walk it almost alone. It is the single biggest gap between quality and visitor numbers in the Roman area.
The Baths of Caracalla
A third-century imperial bath complex that held 1,600 bathers, with walls still standing thirty metres high.
Vast, largely empty, and a fifteen-minute walk from the Colosseum's queue.
The Aventine
A quiet hill above the Circus Maximus.
The keyhole in the Knights of Malta priory door, through which St Peter's dome appears framed by a hedge-lined avenue. Free, thirty seconds, and usually a short queue of people who know.
The Orange Garden next to it, with a view over the city.
Santa Sabina, a fifth-century basilica with fifth-century carved wooden doors — one of the oldest surviving depictions of the Crucifixion.
The Protestant Cemetery
Near the Pyramid of Cestius — a genuine Roman pyramid tomb from the first century BC, built when Egypt was fashionable.
The cemetery holds Keats and Shelley among many non-Catholic foreigners who died in Rome. Walled, shaded, full of cats, and free with a suggested donation.
Keats's stone does not carry his name, at his own instruction.
The Appian Way
The ancient road, surviving with its original basalt paving in stretches, lined with tombs.
The section beyond the catacombs is closed to most traffic on Sundays.
Best by bicycle, rentable at the park entrance, since the interesting stretch runs several kilometres.
The catacombs along it — San Callisto, San Sebastiano — are guided and ticketed.
Underground and stacked
San Clemente — three levels: a twelfth-century church, a fourth-century church beneath, and a first-century Roman house with a Mithraic temple beneath that. A small ticket for the lower levels.
The Case Romane del Celio, Roman houses beneath a church on the Caelian hill.
The Crypta Balbi, part of the National Roman Museum, showing one site's layers from antiquity to the middle ages.
Neighbourhoods
Testaccio — the food quarter, built around Monte Testaccio, an artificial hill made entirely of broken Roman amphorae.
Garbatella, a 1920s garden-city district with courtyards, on the metro.
Coppedè, a small district of fantastical 1920s architecture that looks like nothing else in Rome.
The Jewish Ghetto, one of Europe's oldest, with its own cooking — the fried artichokes are the dish.
Practical
Almost none of these need booking.
Ostia Antica and the Appian Way each need half a day; the rest fit around the main sites.
The Baths of Caracalla and Ostia both have no shade — morning or late afternoon in summer.