I spent a month in Italy last September/October. One week in Venice, one week in Florence and then on a tour for 2 weeks. Had a great time,
Lots of things to see in Venice and the surroundings islands. I especially enjoyed the secret tour at Doges Palace, I asked the receptionist at the hotel to make the reservation for me the day before, it cost 11 euros I believe. You get to see where the Doges spent their lives in secret rooms. I loved going to Murano and Burano, very old world looking towns.
In Florence I rode the city buses around since it was raining most of the time. Seven days was a bit too long there but then I was alone and wasn't feeling too adventurous.
I would never miss seeing Pompeii. I really enjoyed it. I wandered out of the ruins and had to ask farmers in their fields how to get back in to meet my group.
I am sure you will have a great trip. I took the train from Venice to Florence, and then from Florence to Rome, was about 20 euros for each trip. I also used Frommers Italy on $70 a day which was a very good useful book.
Miriam - Oregon