Eat the gelato.  See the Coliseum, the Pantheon, the Vatican. Eat at La Penne D'Oca. Drink the water from the Barcaccia Fountain. Eat at Alfredos.  Stock up on Jordan Almonds (and other incredible chocolate treats) at Chocolate on Via del Gambero.  Eat the grilled vegetable plate from Cafe Strega. Visit Piazza Navona at night.