ROME — The city’s main square, built on the ruins of a first-century arena, is bubbling with activity. Water burbles from Bernini’s famous Fontana dei Quattro Fiumi, one of Piazza Navona’s three fountains, as artists erect their stalls and display their work. A painting of the Colosseum seems to be a requirement. Meanwhile, the handbag sellers hustle to claim coveted spots along the perimeters of the piazza, their forearms laden with rainbows of bags. The trattorias that line the square also anticipate a busy day. Angelo at the Tartufo House urges tourists to sample “the best gelato in the galaxy,” offering tastes of fresh peach gelato “from my own peach trees” on plastic spoons. A Capuchin monk strides purposefully through the square, seemingly oblivious to the couple kissing energetically on a bench nearby. The day’s first tourists arrive, stopping to gaze appreciatively at the 17th-century Baroque church of Sant’Agnese in Agone and to take the obligatory photo in front of the Bernini fountain. Children are drawn to the buskers already on the scene, especially the “invisible man,” whose hat and eyeglasses seem to float in the air. He offers a disembodied high-five to those who toss a euro or two into his can.