Santa Maria del Popolo stands just inside Porta del Popolo, but the plain exterior hides one of Rome's richest church interiors. The Cerasi Chapel holds two Caravaggio paintings, and the church also has work by Bernini, Pinturicchio, Raphael, and Bramante.
It is an easy stop with Piazza del Popolo and the Pincian Hill. Come when the side chapels are lit well enough to see the paintings properly.