I was in Italy last October. I can tell you the weather is perfect for walking around and such, but its too cold for vacation on Lake Como.
I just returned from 3 weeks in Italy. Milan, Venice, Bologna, Florence and Rome.
IMHO, please, please go stay in Venice a few days and I mean on the island itself. I highly recommend Boscolo Venice. Awesome neighborhood. I did a day trip there in 2011 and hated it. This time 4 nights and loved it.
Lots to do in Florence. You can do a day wine tour, or cooking class (we did both two weeks ago). I also did a wine tour last October and the weather was perfect. This year in June/July it was a tad hot. the countryside is just beautiful outside of Florence. We also DIY day trip to Cinque Terre. You might want to do one or two night in one of the cities there. Catch a separate Cinque Terre train out of La Spezia. BTW, skip Piza. The only thing there is the leaning tower and its way too packed with tourists.
So much to see in Rome as well. I've been to Milan a couple of times, once you see the Duomo and the Last Supper, that was about it for me.
Just be prepared, Italy is expensive for tours. Trains are NOT expensive; but you need to pre-reserved in order to guarantee a seat. We did this for all our city changes online, printed our tickets on line and gave them to the train staff as they come through.