Rome

Rome's specific genius is layered: stand on any street corner and there's a Roman ruin, a medieval church, a Renaissance palazzo, and a Baroque fountain in a 50-meter radius. The Forum and Colosseum get the headlines, but the city's body is in the smaller pieces — the Pantheon (still the world's largest unreinforced concrete dome at 1,900 years old), the Borghese gardens, Trastevere's lanes, the Jewish ghetto and its restaurants, Testaccio market. Eat carbonara at lunch and gelato whenever. The Vatican is its own day.