Hard to pick a place in Italy not worth visiting. Venice, Florence and Rome are
musts. Depending on your interests any of these others are worth at least a day: Padova, Milan, Torino, Verona, Mantova, Modena, Ravenna, Bologna, Pisa, Siena, Napoli, Sorrento/Capri/Amalfi Coast, Bari, Palermo, Siracusa.