Magnificent is the one word I would use to describe St. Peters Cathedral. With the largest interior of any Christian church in the world (holding up to 60,000 people), it is truly a site to behold.  You could very easily spend an entire day here and whether you choose to explore every nook and cranny of beautiful religious artwork and history or just sit in quiet solice you will not be disappointed.  My personal two favorites are the Papal Altar & Bernini’s Baldacchino and Michelangelo’s La Pieta, the sculpture of Mary cradling the crucified Jesus.