My friend and I are planning a very last minute trip to Italy. We are spending the first 3 days in Florence and then want to explore Tuscany, Umbria, and Marche. (We have a little over two weeks). From Florence, our plan is to stay either in Arezzo or Siena, spend three days there exploring the city, and taking either bus/train for small day trips to other areas. After those three days, we will be renting a car for the remainder of our stay. We love art, music, architecture (and wine of course!). My question is what city would work best for our needs and would be easier to rent a car from and get on the road without too much of a hassle. Any suggestions about this question or even the trip are greatly appreciated!

Hmmm ! Very broad topic--hard to summarize.
I would bus to Siena and spend 2 days. Then, get your car and stay in 3 other locations over the next 9 days. My choices would be Montalcino, Assisi, and Urbino. Get a good guide book to fill in the details, and spend time at slowtrav.com for hotel reviews and more. Have fun !

Arezzo holds a huge antiques fair the first weekend of each month that takes over the streets in the historical center of town. You may want to check your dates, either to avoid or take advantage of it.

1artzmama,
E-mail me and I will send you some day trip suggestions in Tuscany.

Sienna has a lot of traffic around it and we did not really enjoy it as well as other cities. Couldn't wait to leave. Haven't been to Arezzo, but Lucca was a neat town.

I like bobthenavigator's idea! Nice locations, not too far away from each other, great bases for a wide (but not too wide) area.
A car is a must if you wish to visit Le Marche: great drives and great small hilltowns.