Ciao Arlene2,
Do you arrive into Florence or Pisa? From either, you can use bus or train to get into the centers of both towns and then travel around the area. Your choices might be limited to the larger towns trains reach, but you will still be able to see a lot!
You should dedicate at least 2 full days to Florence, at the very least. The center is small so you can stay pretty much anywhere in the center and still be walking distance from the train station. But depending on luggage, you might want to stay closer or catch a taxi if the suitcases start getting heavy ;-).
You could also base your entire week in Florence itself since many trains depart from here... so you can do day trips to places rather than pack up and carry luggage. Another option is just divide your week in two, with 4 days in Florence and 3 days in Siena, using that as a base from which to go to San Gimignano and even Monteriggioni (by bus on the way to San Gimignano). Arezzo is actually easier to visit from Florence rather than from Siena, just because of train schedules.
Either way, these are the places I'd recommend visiting during your week, in addition to Florence:
- Pisa and Lucca - you could do a day trip from Florence as they are both west of Florence, you could do one in the morning and the other in the afternoon. Or go to Pisa in the morning, then head to Lucca for the rest of the day and spend the night there.
- Siena - can be reached by train but also by bus from Florence (bus drops you off closer to the center).
- San Gimignano - take train or bus to Poggibonsi and then take bus to San Gimignano
- Arezzo - beautiful town and very easy to get to by train from Florence.
How does it sound?