Marnie,
I have to agree with Scott in that it's a good idea to plan your travel between locations as efficiently as possible. Even though the distances are fairly short between all the locations you'll be visiting, two weeks is not a long time so not a good idea to waste any time.
One point to mention is that for driving in Italy, you'll need a mandatory International Driver's Permit. Visitors can be fined on the spot for failing to produce one of requested. This must be used with your home D.L. and each driver must have an IDP. You can obtain these at very reasonable cost at any AAA office (Passport-sized photos are required as well). The IDP will include information on European road signs as well. Be sure to make note of those concerning parking.
As Debra mentioned, it would be a good idea to do some research on ZTL zones. These are becoming more prevalent in many cities in Italy (especially Florence) and driving through one of the automated cameras will result in a fine (€100+ each), which are typically received several months after you return home.
You might also consider taking a GPS unit, as well as a good Map for backup (Michelin). I've found that GPS units aren't infallible but they can still be enormously helpful.
I'd suggest spending at least one night in Siena, as it has a somewhat different atmosphere at night after the "day trippers" have departed. Be sure to observe the dress code if visiting the Duomo there, as I've found they're fairly strict (shoulders must be covered regardless of gender).
For travel from Florence to Siena, Bus is a better option as it drops passengers in the main part of town rather than the bottom of the hill which is the case if travelling by train. It's quite a pleasant trip by Bus and not appreciably longer than by train.