Popping into churches — many of them nondescript on the outside — you'll find sumptuous art treasures inside. The wonderful thing about seeing art in churches is that it's in situ — not hanging on museum walls, but exactly where the art was designed to be enjoyed. Thoughtfully stringing together a series of lesser-known churches can be an art lover's delight. Another even less-visited church, Santa Prassede, is decorated with the best Byzantine mosaics in Rome. Another gem awaiting those exploring Rome's back streets is the Church of St. Peter-in-Chains.