Ciao fdimarco,
Where will you be coming from? Because looking at travel deals you might find good deals for flying into one particular city over another for that period.... end of June - beginning of July is already the start of the high season so my first suggestion is to start looking for deals for airplane tickets to find good deals... in any case, start from there.
Then from that you can start planning how many days to spend in each place.
Looking at the places you want to visit, I'd likely say flying into Rome would be the most ideal choice so that you can start furthest south and then move north. You could fly out of Venice at the end so that you don't need to return to Rome, but if it costs too much, you could also break up your time in Rome to a few days at the start and a few days at the end so that you're in Rome to fly back out.
My suggestion right now would be:
- 2-3 days in Rome
- take train to Florence, use that as a base for 3 days, using one of the days to take the train and do a day trip to Pisa (just 1hr ride away) and to Lucca.
- rent a car and leave Florence to spend a week in Tuscany at a villa or farmhouse and enjoy the countryside. Chianti is an ideal area to stay in and use as a base since it is pretty central in the area for going out and about without driving too much.
Here is a
7 day itinerary I suggest for Tuscany... if you don't count Florence, then you have a few more days to enjoy additional towns in Tuscany such as Volterra, Monteriggioni, Montepulcinano.
After Tuscany you can return the car rental in Florence and then continue on to Venice by train. I'd recommend 2 full days there - be prepared, Venice is crowded pretty much year round.
Take a look at the itinerary, start looking at plane fares and let me know how I can help further! For places to stay in Florence or Tuscany, you can start browsing through these two sites and see which ones you like best:
http://www.florenceaccommodation.com/
http://www.tuscanyaccommodation.com/