I agree with others that with a VERY short 12 days to work with, skipping the Amalfi Coast and Venice would be prudent. There are enough wineries to visit in Tuscany, Umbria and the Valpolicella area near Lago di Garda.
After reading your post, the first thought that came to mind is that winery visits and rental cars are not a good combination, as the penalties for impaired driving are severe. A brief excerpt from Wikipedia....
"From 0.05% to 0.08% (€500-2000 fine, 3–6 months license suspension), from 0.08% to 0.15% (aggravated, €800-3200 fine, 6–12 months license suspension, up to 6 months imprisonment), over 0.15% (aggravated, €1500-6000 fine, 1–2 years license suspension, 6 to 12 months imprisonment, vehicle seizure and confiscation)"
If you're planning on sampling the wares, you might be better to base in Florence or Siena, and take tours of the wineries. I believe there are some listed in the RS Italy guidebook.
It's also important to note that for driving in Italy, each driver listed on the rental form must have the compulsory International Driver's Permit, which is used in conjunction with your home D.L. These are valid for one year, and easily obtained at any CAA/AAA office (two Passport-sized photos required, which may be provided by the issuing office). Failure to produce an IDP if requested can result in fines on the spot!
You may also want to have a look at some of the other posts here concerning the dreaded Zona Traffico Limitato (limited traffic) areas that are becoming increasingly prevalent in many Italian towns & cities especially Florence, which is almost saturated with automated ZTL cameras. EACH PASS through one of the automated Cameras will result in a €100+ ticket, which you won't know about until several months after you return home! This website provides more information - http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm