For beauty & photography, Piazza del Popolo delivers.
A vast, elegant ceremonial square with an ancient Egyptian obelisk and the 'twin' churches — grand, but most people cross it without stopping. The reason to actually pause is hiding on the north corner: the church of Santa Maria del Popolo holds two Caravaggios (the Conversion of St Paul and the Crucifixion of St Peter), free to walk in and see. Climb the steps to the Pincio terrace above afterwards for one of the easiest great views in Rome.