With a full day to stroll you may want to wander from the ancient quarter around Campo de Fiori, where you’ll stroll past Piazza Farnese and the Palazzo Spada, across the Tiber to the young-again and hip Trastevere and then up the Janiculum Hill.
Along this route you’ll find all kinds of architectural gems. Plan to enjoy lunch in one of the jewel box restaurants you’ll find along the way. And for dessert there is always the local gelato shop.
As you climb the Janiculum Hill you’ll be treated to some lovely views of Rome and the Vatican.