Planes, Trains Automobiles (+ Busses?) from Florence to Montepulciano via Siena then Venice

You could rent the car at the airport (rental cars open on Sundays) when you arrive and drive to your agriturismo on Sunday afternoon. Picking up the car at the airport might cost a few bucks more, but if your rental is only a week, maybe the difference is not that dramatic especially considering that you will save the 40 euro or so of taxi to Piazza Stazione + bus or train to Siena. You will not be driving inside Florence at all because you would very easily take the long way by getting on the A1 freeway at Firenze Nord (take direction for Roma) and exit at VALDARNO for Montepulciano.
That is what I would do anyway.

If you don't like the idea of driving after a long flight, I wouldn't go to Siena that day. I would take a taxi to a Florence hotel, and pick up the car the next day from a city location in Florence, then drive to Montepulciano the next day. I can provide directions and pick up locations based on which car rental company you want.

To rent a car, go to a consolidator:
www.kemwel.com
or
www.autoeurope.com
(they are both part of the same corporate umbrella but check prices in both website as prices may vary).

If you are going to Venice from Montepulciano, I would not bother to return the car in Tuscany and take a train, I would drive to Venice and return the car upon arrival at Piazzale Roma. From the drop off location there will be plenty of vaporetti (bus boats) or water taxi (fancier but pricey) that will take you closer to your Venice accommodations.
The drive from Montepulciano to Venice is largely on freeways. Since you already have a car, use it, that way you don't have to deal with carrying luggage on trains and spending over 50 euros per person on the train ticket. The gas and tolls on your car will be half that.

Thanks Roberto, That is especially helpful on the Montepulciano to Venice hints.

I still think that we would like to go to Siena the first day as we have much more interest and see a better fit for what we are looking for likely than Florence.

I think we are set on going to Siena for one night and catching the rental car the next day. Any thoughts or experience with the bus vs the train from Florence airport to Siena where we plan to stay in a hotel near the San Domenico church.

Your question asks about various modes of transportation including planes to Montepulciano.

I'm sure that after landing at Florence you could charter a helicopter to fly over Siena and then pop you over to your agriturismo.

As money seems no object, you could then have a driver bring you the rental car at the agriturismo.

Happy flying...

While the helicopter and chauffeur are fun ideas money is unfortunately a factor.

The "planes" part is a reference to questioning the wisdom of traveling by car the day I land after flying all night with no Italian language skills on jetlag.

Just hoping to give a nod to John Candy and not live out the movie.

But thanks for the creativity!

Still hoping to hear from someone who knows if the bus from the airport in Florence to Sienna is a bad idea vs the train with luggage (and if not how to find the schedule for buses on a Sunday).

You can check Rome2Rio.com for comparison of train vs. bus on Sundays (there are links to bus and train companies).

I think the train may be more comfortable than the bus but some people prefer buses; from the train station in Siena you can take a taxi to your hotel - if you have more than just a carryon and personal item for each of you, I would take the train and then taxi.

You seem worried about your luggage being unmanageable, so do be sure to pack light and make it manageable before departing home! One airline carry-on sized bag per person is large enough, whether you're traveling for 2 weeks or 6 months. You will appreciate packing light in Venice, as well, where walking to your hotel may include bridges with stairs.

J & A, I'm one of the few (I think) who prefers taking the train rather than the bus from Florence to Siena. One of our daughters and I took the bus from Florence to Siena in 1998 and found the curvy roads and speed rather sickening. Since then I've taken the train several times and have no problem getting into Siena from the train station. I have also read of an underground passageway to get from the station into Siena which I plan to take next time Siena is on my agenda.
I see you are from the Palouse. You might be interested in our Travel Meetings in Moscow. Many of us on the Travel Forum who live in the area get together to share travel tips and stories. We'd love to have you join us! Our next meeting will be 21 March. Private Message me and I'll give you all the details if you're interested.

J&A. In your shoes I would probably rent a car on arrival (if Sunday, then the airport is the only option) and drive to Siena or Montepulciano from the airport. It's 100% freeway right from the get go, therefore you don't have to deal with any Florence driving. Driving on the freeway is no different than driving a freeway in America.
Bus service to Siena is limited on Sundays. There are no direct buses from the FLR airport to Siena. You need to take a taxi or the "Volainbus" bus from the airport to the Santa Maria Novella station. From there you can take a train to Siena or from the bus station, practically across from the train station, you can take a bus. As I mentioned the buses are limited on Sundays. The Rapid bus Florence-Siena timetable is below. The Sunday and holiday schedule is on page 4 (Festivo=Sundays+Holidays):
http://www.tiemmespa.it/var/ezdemo_site/storage/original/application/a9cb26924cf7edbe74c346d41aee586e.pdf
There are also Non Rapid buses (called Ordinario=Ordinario), but if you take those then you might as well use the train as it is faster.

To drive in Italy you need to become familiar with traffic. I was working for a week on a business trip in Colfax, WA years ago and I must say that traffic in Italy is more intense than in your area. I also recommend that you become familiar with roundabouts, which are very numerous in Italy as they have replaced most street lights with those.
Here is some info about roundabouts from your State of Washington:
http://www.wsdot.wa.gov/Safety/roundabouts/
And here is a list of roundabouts in your State where you can go for practice.
http://www.wsdot.wa.gov/Safety/roundabouts/washingtons.htm
Or you can go to Lewiston and test your roundabout skills here (I read there is a new roundabout here):
https://www.google.com/maps/place/Southway+Bridge+%26+Snake+River+Ave,+Lewiston,+ID+83501/@46.392799,-117.0384,730m/data=!3m2!1e3!4b1!4m2!3m1!1s0x54a1b563d1c8a5eb:0x4a8d09f38180fa02

For road signs in Europe print this for reference:
http://en.wikipedia.org/wiki/Road_signs_in_Italy

Thanks for the feedback Roberto and good to hear you have visited the Palouse (some friends who have been to Tuscany have told us the rolling hills here remind them of parts of Tuscany just without the olive oil)

Thanks for the tips on driving. It is helping me get ready to take on the roads there. I have driven in fairly remote parts of Ireland on the "wrong side of the road" so I am sure once I get used to what Italian road signs mean what it will all work out well.

We have had a bit of change in our initial arrival. Plane schedules made arriving in Pisa at noon on a Sunday more attractive. So now we are inclined to take the train from Pisa to Chuisi - 3.25 hrs with a connection in Florence (as we did find a rental car place locally that will do Sunday evening pick ups in Chuisi at the train station). But we are open to suggestions about driving from Pisa to the Agritourismo we plan to stay at in Montepulciano http://www.lafalconara.it/pagine/2/en/la-falconara .

Thanks all for the ideas, it is helping make the planning and details of travel less stressful.