Buongiorno Maria,
you could choose Sovicille as the base of your trip. It is a small country village really close to Siena (12 km) and to many other interesting places, hence you do not need to change your base. From here you can reach Siena and spend there one day and visit this small city which has a warm Christmas spirit and where you can taste typical Christmas food like 'ricciarelli', 'panforte' and 'cantucci' with 'vinsanto'. The next day you could visit Florence where you can probably find a lot of Christmas events. On Christmas Eve you could go to Abbadia San Salvatore, it is a mountain village where, during the evening, is organized a special and suggestive event (for more info look at:
http://www.cittadellefiaccole.it/le-fiaccole-di-natale/). For the Christmas day we suggest you to decide once you are here what to do when you will arrive in Sovicille because there will be a lot of events organized in our territory, at the moment we are not able to give you accurate information due to the calendar which is not organized yet. During the other days, you could decide to spend your time following these itineraries:
1) You can enjoy the Chianti area, where you could find good wine and visit the characteristic “borghi” Monteriggioni, Castellina in Chianti, Radda in Chianti and Gaiole in Chianti.
2) You can admire the landscape of the Crete Senesi and visit the “borghi” Montalcino, Pienza and Montepulciano.
3) You can explore the Val di Merse (where Sovicille is located), where you can find the Abbey of Saint Galgano and you can see the sword in the rock. On the way to this Abbey you can find the ancient roman and full of history “Ponte della Pia”, which you can cross by foot.
Have a nice holiday!