You're very welcome and hope my suggestions are useful in one way or another. You will definitely pick up some Italian while here, and Italians are quite patient and helpful in letting you try your hand at speaking at least the most common phrases (compared to some of our European neighbors
).
Using
San Gimignano as a base for Siena and Volterra sounds great. I really enjoy Volterra and if you were coming a week or two later I'd highly encourage you to visit it during the medieval festival - it brings the hilltop medieval village back to life in a special way. It is a highly charming medieval town at any time!
As for
Pisa, I highly encourage at least once in a lifetime climb up the Leaning Tower - at 15 euros per person it isn't cheap but it is a very particular experience because you feel your body reacting differently to what your eyes tell you. Most of the sights in Pisa are concentrated in the "Square of Miracles" and if you don't want to visit the cathedral, baptistery and Camposanto or Museo dell'Opera, then just looking at all from the outside can be anti-climatic. It is generally a half-day side trip from Florence. If you feel you don't have the time, you could leave it out but if you can, make the time to visit all of the sights so that you get all you can from Pisa.
Villa Dianella would be a perfect base near Florence, the train station is really close and convenient for getting to Florence and it isn't far from the FI-PI-LI roadway that would make moving around - to Pisa for example - really easy as well. Actually Villa Dianella is a working winery so you can tour their cellars and do wine tasting right there! They've started offering Tuscan cooking classes as well so you might really want to check that out, at least for one meal.
If Pam comes from a scientific background, then she'll definitely enjoy the
Leonardo da Vinci Museum located in the castle tower in Vinci - this is a newer website but only in Italian while
this one represents the same museum but is an older website, before the museum was renovated and expanded but contains much more details and in English.
In Florence you should definitely visit the
Museum of the History of Science, shortly to be renamed as the Galileo Museum. It is currently undergoing some restoration work but that should be completed sometime this spring. By summer you'll be able to enjoy the entire collection, including Galileo's instruments. You can read more here:
http://www.imss.fi.it/
It is too bad you're arriving in a later period to make it to this special exhibition taking place across Florence -
http://www.firenzescienza.it/cont_E.phtml - especially dedicated to science and the collections available here.
Another museum she might enjoy is the "Specola", a division of the above museum, its particular attraction being anatomical waxes. You can read a bit more here:
http://en.wikipedia.org/wiki/La_Specola
The most important museum in all of Italy, after the Vatican museums, is the
Uffizi Gallery, particularly for its Renaissance collections. I highly recommend spending 2-3 hours there and enjoying some of the greatest artistic masterpieces. You will get to see lots of art in churches themselves so you might not need to do any other museums per se but I highly recommend visiting the
church of Santa Maria Novella and seeing the frescoes in there, particularly the ones in the main altar by Ghirlandaio. The
Medici Chapels are also very interesting, if you haven't yet managed to estimate the importance and extent of the Medici power and riches after just one morning in Florence, their final resting place leaves no doubt ;-).
I hope you have fun planning your vacation and let us know if you need some more pointers. It would be great if you could come back after your trip in August and share with us what you end up doing and any great places to eat you discover - they are always highly appreciated by all!