wow - that's a difficult question. There are so many wonderful places. Make sure to visit our sister site http://www.europeportreviews.com/
for lots of first-hand information on the various Ports of Call.
First of all, I would strongly suggest spending at least one extra day in Rome. While most of the world-famous sights are pretty close together, you will be happy to spend some extra time there. In Naples, a visit to the Pompeii or Herculaneum ruins is a very good choice IMHO. Tom Ogg has posted an excellent Pompeii on you own by train
report. My brother and sister-in-law did the tour in May as suggested by Tom and had an excellent time.
One of the highlights of the cruise will be the arrival in and departure from Venice, which is simply breathtaking. Another favorite of mine is Dubrovnik, Croatia, with its old town and wonderful surroundings, which are sometimes kind of hidden - but beautiful once discovered.