I will echo the suggestions to visit the Accademia. While they do have paintings and exhibits that you can see, to see the main highlights (David and the "Prisoners") is a quick and easy visit. It's inside, just a few rooms, and there are benches where your teen can sit if she wants to wait while you look at the rest of the museum. My favorite time is late in the day - as late as possible before close. They'll stop selling tickets and, if you have a bit of patience, you can have David mostly all to yourself.
We were in Florence last week, and the only place we wanted to get into but couldn't was Il Santo Bevitore. If you have a specific restaurant in mind, it's worth making reservations (usually easy to do via email, check their website). Otherwise, if you eat on an American schedule (around 7ish), you shouldn't have a problem finding somewhere.