This is the single most important tour of your time in Rome (imo)! Definitely sign up to go and see the Sistine Chapel, Vatican Museum Tour, and St Peters Basilica Tour

Scavi tour. These are the excavations below St. Peter's basilica that showcase the bones of the earliest Christians. Will fill up far in advance so book your tour as early as possible. Hit the 'book' tab on the top right of the page that pops up when you follow this link.