Driving Tuscany and Umbria

I'd suggest going thru AutoEurope.com or Hertz and picking a car up in a city center location in Florence--on Sansovino. You can turn the car into Hertz office in Orvieto--across the street from the train station. It's a 70 minute local train ride into Rome Termini.
Rather than stay in Sienna, I'd suggest staying in one of the agriturismos. There are hundreds of farms with apartments and rooms rented as secondary income. We enjoyed touring hill towns like Certaldo, Volterra, San Gimignano and Orvieto from a farm apartment. We found Siena very congested and parking difficult--best visited by 1 hr. bus service from Florence.

Thanks, I thought I just read that the closed the Hertz office in Orvieto because there were problems with that location.

David has good ideas. Day trips from Siena are problematic. I prefer to stay in a smaller town or close to a smaller town. In Umbria, Todi Montefalco, Assissi, and Tuscany, pick one. something near Pienza or Montepulciano for the west side and near Panzano for the east side of A1. Orvieto is great too.

Google agriturismo's in Tuscany and Umbria

Any location close enough to the freeways between Florence and Siena is good for Tuscany. Near Colle Val D'Elsa or Monteriggioni is perfect. Don't stay in Siena with a car as it is a bit congested and parking may be problematic if you don't choose the hotel carefully. It's best to stay in a farmhouse or out of town hotel.

Hertz in Orvieto is still open; it was Avis that closed.

Thank you for the replies! Can we do both regions from one town?

You can do portions of both from one place, but I try to limit the amount spent inside the car.
In that case You might want to stay in a location that gives you easy access to freeway 'Siena-Bettolle-Perugia'. That would place you within 60-90 minutes drive from the most famous towns in the Province of Siena or Arezzo (Tuscany) and in the Province of Perugia (Umbria). Having accommodations near that freeway entrances is key to minimize driving time as regular highways are very curvy and slow.
Here would be good:
http://www.fontedelcastagno.it/index.en.php
Check their prices on www.booking.com also.
http://www.booking.com/hotel/it/fonte-del-castagno-casa-vacanza.html?

A little further north of Robertos' location.

http://www.borgorapale.it/

Thank you for the replies. We can secure a reservation at Fonte Del Castagno. My concern is that Sinalunga seems very far from Orvieto area. We will have already visited Pisa and Lucca on our way to Florence.

Should we split our 3 days between these 2 regions somehow and possibly stay in a small town one night? Or would that be a hassle with a car?

Thank you for the replies. We can secure a reservation at Fonte Del Castagno. My concern is that Sinalunga seems very far from Orvieto area. We will have already visited Pisa and Lucca on our way to Florence.

Should we split our 3 days between these 2 regions somehow and possibly stay in a small town one night? Or would that be a hassle with a car?

You can access Pienza and Montepulciano from there and another day take a long drive into Umbria to visit Assisi, Montefalco and Spello. Tuscany is huge and Umbria has a northeen and southern region. Pick a place and sample what is nearby. There are so many beautiful hill towns that you can't go wrong. Umbria is much less crowded than Tuscany.