Last Fall I booked a ten day "girlfriend getaway" on Hilton points. We flew into FCO and stayed on the 4 night AXON certificate at the Cavalieri Hilton (the BEST ever). We spent one night at a convent in the center of Florence (
http://www.fmmfirenze.it/) and then on to the Molino Stucky Hilton in Venice for another 4 night stay. We bumped into several other travelers in the Exec Lounges following a similar itinerary so this seems to be the consensus plan.
There is a Hilton in Florence, but not near where we wanted to be. In the interest of making the best use of our time, we opted for the very modest and clean accomodations of Casa Santo Nome di Gesu. No regrets. From Florence we took a bus for a day trip to Sienna to see the Marble Floors of the Cathedral during the annual unveiling.
Our last full day in Venice, we took a Valpolicella Wine Tour through Aventure Bellissime which included a stop for lunch in one of my favorite cities, Verona.
This was as much as we could reasonably fit in a 10 day trip and we thought it was perfect. Sorrento is still on my wish list but this itinerary was almost too much. You would have to plan at least a three week trip to include Sorrento, Milan or Sicily.