IMHO Florence is possibly the most beautiful city in the world.
The cheap airlines that fly to Pisa do a coach transfer from the airport that is easier than any other option.
If you're staying in the city a car is totally useless - and there's nowhere to park anyway.
Hotels are expensive but a trawl through trip-advisor should turn up some cheap Pensiones that people recommend.
If you're smart, eating out is great and fairly cheap. AVOID touristy restaurants in the most popular areas and go off the beaten track to find small family-run joints where the locals eat and everything is in Italian. Again, trip-advisor can help.
Whether you like art or not, the Uffizi is a must as is the Acedemia (where Michelangelo's David is). I think both are closed Mondays so watch out.
Likewise, even if you're not into Football that much it's worth going to see Fiorentina. Women get in cheaper too(!). There are ticket booths all over town. The atmosphere is great and you can sit in the sunshine with some Gelati.
Learn a smattering of Italian, get a guide book and have a great time.