does this itinerary look good--we could still change it---leave in 3 weeks

I would also add another night in Venice. Did you ever think you might want to play it by ear and stay as long as you like in one place and than book lodging in the next place you decide to see. If you really like a place you can stay longer and if you feel like moving on you can. With internet access you can find lodging last minute.

If it were me, I'd take a night from Monterosso and add it to Venice. Assuming you fly into Venice, the first day while in theory should be productive, may get you with some jet lag. The trip from Venice to the CT is long, so two nights in Venice might turn into just one full day. Three nights will give you two full days after arrival. Otherwise, looks good.

I agree with Ron: Add a night to Venice. It is a magical city. Get away from San Marco and wander. The Dorsoduro is particularly nice and so few visitors get over there, beyond the Accademia. We loved Orvieto! Good restaurants, very quiet at night. Good to have a car though to take some daytrips. If you like to hike, the Rupe, around the base of the hill, is an intersting walk. Buon viaggio!

Another option would be to chose either Florence or Siena for a base and combine those days. Then, take a day trip to the other. It's an easy bus ride. That way you could have 5 nights in either Siena or Florence or take one of those nights for an extra one in Venice. Venice--3 nights.
Siena or Florence--4 nights. The rest looks good!

I agree with Ron, and want to add to watch Sharon said. Unless you have things you specifically want to see in Siena (things that would take up more time) I would stay in Florence for the entirety and then take a day trip (by bus or train) and see the piazza, the museums, etc. I think one day is enough there, and Florence will have much more to see at night!

I agree with Ron that the trip from Venice to CT is long (5-6 hours by train I think). You can get to Florence from Venice in 2 hours, and then from Florence to Monterosso in 2 1/2 - 3 hours. This is what I would do: Venice - 3 nights Florence - 3 nights Monterosso - 2 nights Siena - 3 nights
Orvieto - 3 nights I would get pick up a car after CT to have in Siena and Orvieto. I would an extra night in Siena so you could explore the many hill towns around there as well.

In reference to the rental car, I agree it makes sense to have it after visiting the CT and Florence. I would pick it up at the Florence airport--not the train station. This way you would have it to visit the hilltowns in Tuscany and the towns around Orvieto. Then return the car at the train station in Orvieto. If you have a car while visiting Siena, you could stay in the country surrounding the town. It's so beautiful. If you need suggestions for lodging, PM me.

I'm doing something similar Venice 10 night (photography workshop) Florence 3 nights Siena 2 nights Monterosso 3 nights Rome Siena to Monterosso or visa versa by train is 3:13 with 2 changes. With your itinerary I would do: Venice Florence Monterosso rent car in La Spigga after leaving CT Siena Orvieto return car Rome
I leave late this month too maybe I'll see you in Italy;-)

From all of the posts I've read here and on Frommer's Forum Cinque Terra is going to be jammed with Americans. I would drop a night from Monterossa and add it to Venice. Also, in 2008 I spent 3 nights in Orvieto, which was at least 1 night too many. While it is beautiful it is expensive and the town closes at 9 p.m. Buon viaggio,