Pantheon, Rome

The best-preserved building of ancient Rome, a temple completed around 125 AD and a church since the 7th century. Its huge concrete dome, still the largest unreinforced concrete dome in the world, has an open oculus 9 m wide at the top; when it rains, the water falls straight onto the marble floor and drains away. The artist Raphael and the first kings of Italy are buried here.

There's a small entry fee and timed tickets can be booked online. The Piazza della Rotonda in front is lively with cafés, and the Piazza Navona is a few minutes' walk away.