Perhaps one of the best known of Rome's public squares, the Piazza Navona dates back to the end of the 15th century. Today, it fills with modern people sipping coffees while watching street performers and artists. Cafes abound, and there are a number of shops too, although recent visitors said both tend to be expensive. You'll also find a number of impressive monuments, including one by Gian Lorenzo Bernini (Fountain of the Four Rivers) and another by Francesco Borromini (Sant'Agnese in Agone). 

You can reach Piazza Navona by hopping off the metro at Barberini and walking about a mile west. It is free to visit 24/7.