We've been to Italy twice and didn't bother going to Venice or Milan. Milan is just a big modern city with lots of fashion shopping and nightlife (unless you like that stuff). Venice is an overpriced tourist trap. I wouldn't go to Pisa ever again for the same reason.
- Florence - re-consider going up the bell tower if you were considering it. The view is nice, but spending 2 hours on the stairs was ridiculously hot and unpleasant. Uffizzi Gallery is impressive, but at some point one can experience an art overdose in Italy, so all churches and museums start looking the same.
- Siena is not only beautiful but also very pleasant to walk around and reasonably priced.
- We stayed a week in Sorrento and absolutely loved everything about it. Took a day-trip to Capri and hired a small boat for a private ride around the island. Highly recommended.
- Rome - there's no way to see it well in less than a couple of weeks; so whatever time you spend there it's priceless and unforgettable. Vatican is a must, of course.
- We really loved Northern Italy - Rapallo and Portofino, but not Cinque Terre (thousands tourists everywhere).
My biggest advice not to try and see everything. Pick a couple of targets and spend a week at each if you can.