First trip and feel lost

If you are interested in Renaissance art/architecture/history, I would suggest adding another day or two to Florence – particularly since you may be jetlagged and not very efficient on your first day in Italy.

Spending the remaining time in Rome will minimize your travel time. Orvieto is an easy day trip from Rome – just a little over an hour by train.

And if you don't have your plane tickets, it would be more efficient to fly into Florence and out of Rome. Just select the option labeled as something like "multicity" when you search for a flight.

You could also spend your remaining time in Venice - my favorite city in the world. It's a very easy train ride from Florence and I've also traveled from Siena to Venice by train with no problems. Of course Rome is amazing, too. You might read up on both cities then make up your mind. With 5 days, you could rent an apartment in either place and likely be more comfortable than a hotel.

Both Rome and Venice are great cities and can easily fill 5 days.
Renting a car and driving in Tuscany is very easy. I've rented in Siena and the traffic is not too bad. A car gives you the luxury of really exploring the Tuscan area and not being dependent on public transport, which is not too great compared with the cities.

zins,

If you plan to visit smaller towns in Tuscany that aren't well served by public transit, having a rental car is the best way to reach them. Siena is not a particularly good town to have a rental car, unless your hotel provides parking (public parking can be hard to find and expensive). If you decide to visit larger cities that are well served by train or bus, then using public transit would be the easiest and most efficient method, especially on the fast trains which travel at up to 300 km/h.

There are some potentially expensive "caveats" to be aware of both with rental cars and with public transit. If you need more information on those, post another note.

Following up on Ken's good advice, if your villa is near the historic city center, you can use buses to visit some of the Tuscan towns (Florence is an hour away by bus, center to center). Then you'd only need a car for a few of those 7 days, to tootle around the countryside. But if your villa is farther away, you'll want the car for the whole stay. Also consider the rental rates. You will probably have more options renting in Florence than in Siena and a longer rental may get you a cheaper daily rate.

After returning the car (in Florence), take the train to either Orvieto for a night or two or straight to Rome. Orvieto can be a day trip from Rome by train, but it is much better as an overnight stay, so you can enjoy the town in the early morning and late afternoon/evening hours when the many day-trippers are gone. Maybe you could even drive to Orvieto and return the car there.

Where you spend your last night depends on what time you have to be at the airport for your flight home (assuming you can't alter your plan and fly out of Rome). There are frequent fast trains between Rome and Florence, but if you have a morning flight, you may need to spend that night in Florence. Whether you stay overnight in Orvieto (or even go there) may depend on how much time you really have in Rome and how many hotel changes you want to have.

Unfortunately, Italy is an embarrassment of riches when it comes to great places to see. So, it's not just you who feels lost, and it's not just a problem faced by first-time visitors. It really is hard to figure out where to go. Just remember you can't make a "wrong" choice - it's all good.

Do you have Rick's Italy book? He has descriptions of lots of great places. If you are more of a visual person, try watching his videos, since he has lots of ones on various places in Italy. At this link, scroll down and click "Italy" for the list: http://www.ricksteves.com/watch-read-listen/video/tv-show

I do agree with planning to spend more time in Florence itself; for me, the day of arrival is mostly a jet-lagged haze, and while that can be fine for walking around, it's not the way to appreciate Renaissance art. I also loved my day in Lucca and Pisa, done in that order. By getting to Pisa in the afternoon, the tour bus crowds had thinned, the souvenir vendors were literally packing up, and the marble in the Field of Miracles looked lovely in the afternoon light.

It's okay to feel lost, you will get your bearings as you work things out. The roninrome.com website has some great info on navigating Italy (Roma and beyond).

I still have places to see and I've been to Italy at least twenty times (not counting the time I lived in Roma on and off).

You are off to a good start. Make sure your villa host gives you plenty of information.

While you are in/near Siena, take some time for Montalcino.

If you want to visit also Rome, it would have made sense to fly into FLR and return to the U.S. from the Rome airport (FCO), that way you would have saved the train fare back to Florence. But if you already have your tickets don't bother to change them because the fee for changing it charged by the airline would be more than the 45 euro train fare.

If you want to visit smaller towns from Siena, I would definitely rent a car. Also you said it's a villa, therefore I suspect it might be even outside of Siena, in the ountryside, therefore a car is a must.

This is a list of famous towns or villages near Siena:
Chianti Hills (Castellina, Greve, Panzano, etc.), San Gimignano, Volterra, Certaldo, Colle Val D'Elsa, Monteriggioni, Pienza, Montepulciano, Buonconvento, San Quirico, Montalcino, Bagno Vignoni, and many others (see also www.borghiditoscana.net for ideas).

The region of Umbria is also close to Siena. Famous towns are: Perugia, Assisi, Gubbio, Spello, Spoleto, Orvieto, Todi, etc. I think in 7 days in Siena you could hit some of them too if you have a car.

If you intend to visit Rome for a few days, do so after you return the car. You don't need a car while you stay in either Rome or Florence. Rome is only 90 min from Florence with the high speed train (but at least 3 hours by car). Orvieto is very close to Rome (70 min by train).

If at all possible, keep the car for the drive to Orvieto it's only 1h30m. then train into Rome.

Better yet keep the car until Chiusi and then train into Rome.

Wife and I have always checked our bags then taken double-decker city tour bus. Great way to get orientated in Rome. Number of hotels walking distance from main terminus. These bus tours depart from the train station. Have fun!

We absolutely loved Lucca (walled Medieval city) and San Gimignano. Both gorgeous and walkable. And also a little less crowded then some other Tuscan cities (although October should be a terrific time to see Italy with fewer crowds).

Zins, you have been given some excellent advice> Italy has some excellent public transportation options, but a car works best for exploring the small towns. Make sure that you read up on "ZTL's" before you drive into any major city in Italy, or a year later you could be paying hundreds of $$$$ in traffic fines..

Don't forget that there is excellent bus service between Sienna and Florence; we based ourselves in Sienna and took the bus in to visit Florence.