I'm posting this from Todi, in Umbria, a couple of hours south of Florence (1/2 way between Rome and Florence).
Florence is a wonderful town, and I agree with everything said about Pisa.
Stay away from eating/drinking in the 2 main squares as they are very expensive. There is always someone playing in the square inisde the Uffizi at night, and the acoustics are wonderful, it's quite special. The gallery is spectacular as well. A replica of Michaelangelo's "David" is in the Piazza Della Signora which is right next to the Uffizi, and the Ponte Vecchio is right around the corner, all 5 minutes from the Duomo which is superb.
There is a Novotel near the Florence airport, which is relatively inexpensive, and has a regular shuttle bus which goes to the train station (10 minute walk to all the big sites).
Siena is special, well worth the trip. San Gimiganano is also really cool, but "smallish". Lucca is also not far away (between Florence and Pisa) and is a really neat place.
Further afield (and close to where I am now), Assissi is super, as is Orvieto, and Todi. If you want further info on these places, just ask.