I mentioned the Solstice because the OP did not reference and specific cruise. Celebrity is owned by RCI and most consider their ships a bit classier and more upscale than RCI. We enjoy celebrity because it is a more laid-back product than RCI which is very activity oriented. A day in Paris really should depend on your personal interests since it would be easy for me to plan a week in Paris. For one day in Venice we would recommend just exploring Venice on foot and certainly walking between St Marks Square and the Rialto Bridge which is a route loaded with shops, cafes, and great atmosphere. If you have an interest in Murano, you could always grab a vaporetto and go over to that island for a couple of hours. As to Rome, unless you are in the city on a Sunday we think the Vatican is certainly worth while (this can take up to half of your day). Otherwise it depends on your interest in ruins (Palitine Hill area and Colliseum). If ruins are not your thing than you might want to walk over to the Spanish Steps and Tervi Fountain area. Our personal favorite spot in Rome is the Piazza Navona, where we sometimes spend hours just sitting at a cafe) but this might not fit a hurried one-day itinerary.