Makin' Florence Rock!
Buongiorno,
So the first thing to remember is the December 8th is a National Holiday - and though restaurants will be open, many shops and such may have varied hours.
My experience with my 20-year-old niece - sometimes you never know what is going to spark their WOW factor, but let's give it a try.
Though the popular sites make a lot of sense to us (the churches, the museums, and national monuments) they might not make so much sense to her - you could start by suggesting that you are going to do a walking tour, on your own, and have her look through these themes itineraries (
http://www.visitflorence.com/itineraries-in-florence/). A few that might spark interest for her are the spots from Dan Brown's book Inferno or following Michelangelo thorough Florence. I particularly like the city wall walking tour, but the idea of the city "as it was" always intrigues me.
If you are going to look at doing the museum route then definitely book a guide who will run you through the line - waiting in long lines can be a downer. For example, the Duomo is amazing - but the line on the 8th could be daunting (the holiday celebrates the Annunciation so churches may be busy). I did the Duomo rooftop terrace tour - so it is something really different from most tours (exclusive) and it gets you into one of the most iconic dites:
https://www.visitflorence.com/florence-tours/duomo-terrace-tour.html
http://tours.visitflorence.com/en/13618/travel-tips/Skip-The-Line-Museums-with-Guided-Tour/ttd
With kids it's not always what they see as much as how they see it and who they see it with - so maybe an activity would be a great addition to a day of sightseeing, for example, a cooking class?
https://www.visitflorence.com/what-...es/small-group-cooking-class-in-florence.html
Sometimes mixing the local sites with the local flavors?
https://www.visitflorence.com/florence-tours/art-and-food-walking-tour.html
And it might really help if she had a few ideas on her own, maybe if she read through a few of our suggested itineraries for two and three days, she would come up with a list of places that you two could go and search out on your own? (but I would still finish the day with a cooking class!)
http://www.visitflorence.com/itineraries-in-florence/two-days-in-florence.html
http://www.visitflorence.com/itineraries-in-florence/3-days-in-florence.html
The only thing I don't really suggest (unless she really wants to) is trying to leave the city to see Pisa or Lucca... one day in the city barely scratches the surface, leaving it early will just add to your travel time and not let you absorb its natural beauty.
Buon Viaggio,
Donna Denise