Pantheon

It is simply one of the marvels of Roman architecture and history, and you must include it in your visit. Nearly two thousand years old and still the largest unreinforced concrete dome ever built, with a hole in the middle that has never been closed. Raphael is buried inside, against the wall on the left, as well as Vittorio Emanuele II, the first king of unified Italy. Despite its pagan origins, the Pantheon is actually a basilica: it was consecrated as the church of Santa Maria ad Martyres in 609 AD. It’s still a working church, with an altar and a bishop’s chair, and masses are celebrated inside so you can sit and contemplate. You don’t necessarily need to buy tickets in advance; I never did, and the machines at the entrance worked fine.