Just remember you are a long way from Rome when you get off the ship, be sure you include transfers in your tour.
We did the organized tour with Princess, we went down the "main drag" of the Vatican Museums, Sistine Chapel and St. Peter's followed by a little time in St. Peter's Square. Not enough time in the museum, and we would have gone to the dome of St. Peter's with more time there.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014
Star Clippers: Royal Clipper 2015