December in Tuscany
Buongiorno,
So if you add Rome to the equation, you will be in Tuscany 3 days. One of which should be dedicated to Florence, and since you are close you could include Lucca and Pisa on another day (both are easily reached with the train if you want to ditch the car.) And then you could do another day enjoying the spa at Montecatini combined with a half day visiting the countryside and maybe a vineyard.
I would catch the train from Florence to Rome, overnight in Rome and then Rome straight back to Milan. The train is faster, more economical (no worries about filling up with gas, parking and ztl zones) and a lot more relaxing.
However, I would add that as great as Rome is - you are spreading yourself just a bit thin. Especially since in the winter daylight hours are considerably shorter.
Buon Viaggio,
Donna Denise