Academia gallery and Ufizzi. Also, Piazzalo Michelangelo had an amazing view of the city. It offers a great view of the city. The Great Synagogue is also a nice tour.
+1 to all of these (although we only did the one with the David, it is really a site to behold), we didn't have time for both. The Great Synagogue is stunning. It is open every alternating half hour, with the museum upstairs open the they half hour. I wouldn't wait to see the museum if you get there at the beginning of a half hour.
I'd also consider doing a walking tour of the city (although you'd have to wait outside the Duomo for a little while)