Second time in Rome so we chose to do Rome on your own - $79 which was quite expensive. My advice would be to use the train which I believe is only 7 Euros. That way you can come back when you like. We really enjoyed our day but by 3pm were ready to head back to the ship.
Beautiful little place and would highly recommend just getting off the ship and having a walk about. We found a little bakers and went and sat in the square to eat our lunch. Lovely and clean with nice little shops and squares.