On any of my trips to Rome I must visit Saint Peter's. It doesn't matter how many times that I've been I always seem to find something that I have missed on my last visit.
I've also found it wise to go very early (Saint Peter's opens daily from 7am until 6pm) before the crowds and tours arrive. I avoid Wednesdays for the Pope's service when the crowds are enormous and don't be fooled into buying entry tickets for Saint Peter's as it is free to get in.
Saint Peter's Basilica was built over the supposed burial place of Saint Peter himself and there has been on a church on this site since the 4th century AD. Over the centuries Saint Peter's became decayed so during the 16th century Pope Julius II commissioned the construction of the new Basilica.
The inside of Saint Peter's is immense, decorated with marble, statues, gold stucco and mosaics.
When you first enter head to the right and you will find Michelangelo’s marble statue of the Pietà, his only signed work. Unfortunately it is protected behind glass after it was damaged by a crazy man with a hammer in 1971.
The Chapel of the Sacrament holds the only canvas in the church, the Trinity by Pietro da Cortona.
The Treasury can be visited for a fee via an entrance in Saint Peter's and is worthwhile to see church ornaments and papal mitres and jewelery.