I spent 2 weeks in Tuscany in 2000. We rented a villa in Sovicille, just outside Sienna. We rented a car and took day trips to visit the hill towns. The last 3 days of our trip was spent in Florence, where I proposed to my wife. Suffice to say it was a fantastic trip.

I have my list of recs for that trip somewhere, I'll need to find it if you're interested. Granted, I don't know how much time you have in Italy though.

Highlights.
Food:
1) Drink as much wine as you can. You pay by the glass so you don't have to worry about buying the bott.e
2) Order local specialties, even if it's daring. My best pasta was a wild boar pasta. 2nd would be the fried goat at Buca San Antonio in Lucca. FANTASTIC. I also had horse in Parma, but that's another story (but good).
3) Food tends to be better in the small towns, in small out of the way restaurants. Big towns like Florence can be a rip off, just like eating at the airport or Fisherman's Wharf.
4) In Florence, try the steak, especially steak balsamico at Aqua del Due. Better than the Florentine, I even went twice. It was even shown on Rachel Ray's $30 / day show.

Towns.
1) Sienna. Great duomo (church), though you'll get more than enough in any relatively large town.
2) San Giamignano (spelling)? The "Manhattan" of Italy, GREAT white wine.
3) Asissi. Go see St Francis of Assisi, I'm Catholic, but not religious. This was very moving for me.

Sites:
1) Statue of David in Florence. It's the only thing in the museum, but worth it. It looks like real flesh.
2) The Main museum in Florence (now I forget the name).

We tend to use guides like Go Italy and Frommers. Fodor's tends to be too fuddy duddy for us.

If you want more info, PM me. I loved that trip, can you tell?