5 days between Venice and Rome

I'd certainly add a night to Venezia. Two is just not enough as it only gives you one full day to tour. Since Firenze is only about seeing the David, can you put that night toward Venezia, take an early train from V to F, see David, rent a car after lunch, and head to Siena? Driving is not bad as long as you watch the ZTLs and don't ever try to drive in Rome. (I only drive to get OUT of Rome!) Siena can be a parking nightmare so stay somewhere peripheral that has parking. We stayed here in the city and they had one garage they let us use. I know I have read about a couple of hotels that are outside the ZTL with parking but close enough to walk in. You can certainly walk to/from the city center in 20 minutes or so even from outside the walls. We are staying near Montepulciano for the first time next weekend at Poggio-Etrusco. I can let you know how it is. We usually stay in Montalcino, which we like very much, and we always do the drive between the two towns, stopping in Pienza.

With this plan, I'd pick up the car in Firenze and drop it off in Orvieto or Chiusi.

Food:

In Venezia we are working our way through the Telegraph list Venezia

In Firenze I had the BEST pork of my life at Boccadama, overlooking Santa Croce. Also really liked Osteria Porcellino near P.za della Signoria.

In Siena we liked Trattoria La Tellina very much.

I have a list for Rome that I send to friends all the time. It is posted here.

In addition to Laurel's great recommendations, I like Arezzo, Anghiari and Sansepolcro as well as Montalcino. If you have time for a stop in Umbria, Todi is beautiful. I can recommend a small hotel there.

In Siena, very near the Ovile gate, is Hotel Minerva, with covered garage parking onsite. Ten minute walk to the Centro Storico.

A whopping 2 nights in Venice - one of the most unique cities in the world??? Hopefully you'll rectify that AND you'll choose to stay on Venice Island...not in Mestre.

The sentiment to add a night in Venice seems pretty strong. It hasn't been high on my list but people seem to love it so I wanted to see for myself. I'll have to consider it and I have been thinking of making Florence a day trip.
Alternate plan B..... Train to Florence, stow bags at station, see David and have lunch, train to Siena. Our hotel is near the train station in Siena. I had then planned one day in Siena without a car, the next day take a day trip North and the next morning drive South to Montepulciano.
Thanks for all your input so far. Any suggestions on Scenic routes and stops along the way are also appreciated!

Definitely stay IN Venice, not outside the city. Wander and get lost. If you're not looking for fancy, The Hotel Astoria, two stars, was great and a very good location not too far from St Marks. I find it works out better to ask for the rooms with two twin beds, even for a married couple, than one bed that usually ends up to be just a double, too small, too old, not comfortable. We got a twin bedded room at the Astoria that was much larger and more comfortable than our friends in their tiny, cramped, double bed room.

Don't miss San Gimignano in Tuscany! At least make a long lunch stop there and see the view from the top of one of the towers. Siena is great, you'll love it. Driving in Italy is more challenging than France, but we've done it many times. Pack your patience. Florence is a confusing city to drive in the central area, and very little parking.

I'm with the others, spend another night in Venice. Depends on the weather and flooding, number of tourists, etc., though. Venice is sometimes best in winter.

CIAO! Enjoy!!

We had just two nights in Venice and thought it was more than enough time to see/experience the city -- I guess it depends on how in-depth you like to get when you're traveling and really get to know it or get a feel for it. And don't get me wrong, I absolutely loved it, but I wouldn't trade it at the expense of more time in Florence or Tuscany. I think the time between those two depends on what you want to experience, as they are so different -- the larger city with the world-class museums and art or the smaller hill towns with amazing wine (and both places have fantastic food that I still think about two years later). Our favorite was Tuscany, but had no regrets spending a few days in Florence either -- you can probably see a lot in two. And then just pick one town in Tuscany for your remaining time (you can hit places on your way out of Florence and on down to Rome too).