To add to Slawecki's comments. Read up about ZTL (Zona Traffico Limitata) as they are new relative to your last visit. Essentially, many Italian cities have these zones and restrict traffic in them with varying criteria. In Firenze, you can rent and leave the zone but be careful about your route. If you cross back into it, your license plate will be captured and you'll get an expensive ticket via mail and an administrative charge from the rental company. You can re-enter to go to your hotel but they will need to call in your plate #. Siena has a ZTL too which restricts where you can go.

When booking a rental, look closely at the deductibles. Personally, I go with Auto Europe as their deductibles are lower than Hertz and the buy down to a zero $ deductible is a lot cheaper that Hertz's Super Cover and their liability coverage is great. I have a gazillion Hertz points but the insurance cost is so high in Italy that I don't bother with them. Also Amex doesn't provide secondary coverage in Italy although some Visa do. I'm picking up a 2 week AE rental @MXP in a few weeks with $0 CDW & theft for $860 (manual and size for 2 adults).