Other tips are all good, but I would add these;
- Check with you insurance agent and to see if your rental car will be covered by your auto insurance. Also check with the credit card you will be using to see if it covers insurance on a rental in Italy.
- Check your medical coverage as well.
- You generally get the best exchange rate on an ATM, but beware any foreign transaction fees you card may charge. Worst exchange rates are found in the hotels and at the airport and other money changers. You will do better using your credit card when possible for purchases rather than exchanging cash because your cc company generally offers the best rate on purchases.
Lastly, I second Conway, get the international drivers license from triple A!!! Its only $15.
Have fun Tom!!