We have four nights and that includes Florence, Siena, hill towns etc. I was thinking that was a fair amount of time, but I guess it really isn't. My husband would prefer that we not rent a car. We don't want to be on the "run", we want to be able to enjoy the area and of course prefer smaller towns to big cities. I'd really like to see "David" but don't have my heart set on anything else. What would you suggest??
Your husband is exactly correct about not renting a car. There is enough in and nearby Florence to keep you well occupied for 4 nights. Take public trains and/or bus to Siena...also Pisa is a very fun little day excursion (via train). I would suggest the Residenza Il Villino for your stay in Florence. It's a family run mom and pop B&B with the nicest mom and pop (Elizabeth and Sergio) you'll ever meet. It's also very very close to the Accademia where the David is located.
If you travel point to point between cities. A car is a needless headache.
When you get out in the hill towns that are off the beaten path. A car opens up a lot of options.
Probably the best way to decide is to study different destinations, pick your must sees and see if they are reachable by rail. If so, rail will work fine. If not, you will have to weigh whether to skip it or rent a car.
Adding Siena and hill towns to Florence is probably two days at least with your visit to Florence. You'd want a rental car. Alternatively, stay in Florence and train to Lucca.
A lot of hilltowns are accessible by public transit, either direct bus or train to the foot of town and then a bus up. We actually did the reverse of what you're planning, staying in Cortona and doing Florence as a day trip. Cortona to Florence can be as short as 1 hour depending on the train you choose, Orvieto 1.5 hours, etc.