I agree with doing the Uffizi early in the day. And, as the other reply said: What about the Duomo area. I would actually head straight to the Piazza del Duomo since you're arriving fairly early. If there's not much wait (there probably will be though), pop into the Duomo. Otherwise, climb the Giotto Bell Tower adjacent to the Duomo -- if you're able (and fairly high energy) it's a long but very doable climb to the top of the Bell Tower and you'll have a fantastic overview of the city (and it's usually little to no wait to climb the Bell Tower). Re: the Uffizi and the Accademia, definitely MAKE RESERVATIONS! As Rick Steves says: to wait in line for several hours vs. entering fairly quickly with a reservation is just stupid! If the Uffizi is on your must do list, have a reservation (Rick's guide books give you all the info as to how), and have a game plan as to what you want to see. Otherwise, I would skip the Uffizi because it's just too much to do in such a limited time frame. The Accademia (again with reservation) is, IMO, more of a must see because of Michelangelo's David. I'd spend most of the rest of the time exploring Florence on foot, especially the area around Santa Croce Church (and inside the Church). Another museum that is much easier to do than the Uffizi is the Museo del' Opera del Duomo -- it's behind the Duomo, usually not crowded and contains the original Ghiberti doors to the Baptistery (the ones on the Baptistery are Japanese reproductions) as well as one of my favorite statues, Donatello's Mary Magdalene (it looks like 20th century art!) as well as a spectacular Pieta of Michelangelo. If you can, cross over the Arno at the Ponte Vecchio and dip your toes into the Oltrarno, the area on the other side of the Arno. If you want a nice, sit down lunch, my favorite restaurant, Casalinga, is between the Pitti Palace and the Piazza Santo Spirito.
If you have a little time before you catch your train out of town, stop in Santa Maria Novella church right near the train station.