I visited Italy on a cruise back when I was 16 and have been chomping at the bit to go back ever since. The second half of the big Europe trip I've got planned for this fall is in Italy - 2 days Venice, 3 days Florence (one will be spent hiking the Cinque Terre), 4 days in Rome and 2 days in Sorrento. We were going to spend half a day in Milan, but I couldn't get tickets to the Last Supper, so I decided to spend extra time in Venice instead. Here are my impressions of the major cities from one short trip over a decade ago and my planning over the past 9 months.
Venice - Stay on the islands, not the mainland. Everything except the lagoon islands (Murano, Burano Torcello) is in easy walking distance. Vaporetto trips are expensive, buy a pass. It costs a lot more if you want your gondolier to sing. If you visit in late fall, bring rubber boots. The Bellinis at Harry's bar are overpriced and not that good. Beware the pigeons. This is one city where it's worth it to get a little lost. That being said, buy a good map. Venice's major sights can be covered in a day trip, but it's a very busy day.
Florence - Orange. I recall Florence being the most beautiful city I'd ever seen, but very orange. Once again, the major sights are all within walking distance. This is a museum city, and cannot be done in a single day (especially if you actually want to enjoy the art). Make sure you get reservations ahead of time for the Accademia and the Uffizi, or get the new Firenze pass that just came out this year. This is the place for leather goods. Don't actually buy anything sold on the Ponte Vecchio, it's way touristy and overpriced. Florence is the gateway to Tuscany, and the best place to serve as a central location for visiting Pisa, Siena, Luca, Fiesole and other lovely Italian towns.
Pisa - Can be done in 2 hours. See the field of miracles, then you're done. The leaning tower now has online reservations.
The Cinque Terre - Actually, I don't know much about these. Never been there. DH is the hiker. A park hiking pass/train combo costs about 10E. Hiking north to south is supposedly easier than hiking south to north. There's a sweet local wine that's supposed to be very good, and the anchoives are supposed to be excellent.
Rome - Rome cannot be seen in a day...or two days...or three. The Vatican takes a day, the forum area Roman sites should take a day...the majority of my memories from my quick jaunt to Rome consist of me thinking "OMG, there is just waaaaay to much to see and do here!" There's a Roma Pass (which is nowhere as good as other cities' passes, but still better than paying full price). The Vatican now has online tour reservations (yes, EVERYTHING is now online). Then there's the sites just outside Rome: Ostia Antica, Tivoli (with Hadrian's Villa and the Villa d'Este), Tarquina, Ceverteri, etc. Oh, and did I mention my DH was a Classical Civilizations major?!? I'M SPENDING 4 DAYS THERE AND DO NOT HAVE NEAR ENOUGH TIME TO SEE ROME WITH A MAN OBSESSED WITH ROMAN HISTORY!!!!!
Sorrento - If Florence is the most beautiful city I've ever seen, Sorrento is the most beautiful town. This lovely little laid-back location is the gateway to Pompeii, Herculaneum, Positano, Paestum and Capri. Wonderful pizza, beautiful woodworking, ocean views to die for...and I can't wait to try the limoncello.