I just wanted to chime back in on this thread for a minute and make another recommendation or two in Florence.

In addition to the obligatory Accademia (statue of David) and Uffizi Gallery, I have gone back a few times to take friends through the Palatine Galleries of the Pitti Palace.

The Palace is a 10-15 minute walk into the Oltrarno district on the "other" side of the Ponte Vecchio. It is the royal apartments and art collections of the Medici Family, and the rooms are just jaw-dropping. Rather than a traditional museum collection of paintings, you can really get a feel for how the wealthiest family of the Renaissance lived. A visit to the Palace can be combined with a picnic in the Boboli Gardens, or a walk up to Piazza Michelangelo for stunning views overlooking all of Florence (best photo op. in the city, IMHO).

Secondly, one of my fondest experiences in Florence (which I love, but not as much as Venice and Rome!), is stumbling upon the "Officina Profuma Farmaceutica di Santa Maria Novella," the oldest pharmacy in the world. Dominican friars began mixing curatives in 1221, and opened this pharmacy in 1612 with permission to sell stuff to laypeople: http://www.smnovella.com/english.html

It is very centrally located near the Santa Maria Novella church, but can be easily missed on the street...so ask anyone and they will know it. They sell all sorts of perfumes, soaps, lotions, etc. The potpourri is very affordable and makes great souvenirs, and if you keep it in your luggage during the rest of your trip, all your clothes will smell AWESOME. ^
Honestly, it is worth a short walk out of the way to see this place...very evocative of a time gone by, and walking into it (it is quite large) is one of the most incredible smells you will ever experience (like a Medieval Aveda store!).