Rome

Rome needs 4 days: the Colosseum and Roman Forum on one, the Vatican (Vatican Museums and St. Peter's Basilica) on another, and the rest for the historic center (Trevi Fountain, Pantheon, Piazza Navona). April-June avoids extreme heat and European summer crowds. It's expensive by Italian standards, though more affordable than Venice or Milan for street food. It's the only city in the world containing an entire sovereign state within its limits (the Vatican), and its over 2,500 years of continuous history are unmatched in Western Europe.