Hi Rebecca,
Something I would recommend is doing Siena & San Gimingano together. I personally preferred San G. over Sienna since it was a little smaller. But you can do both of these very easily by bus.
Cinque Terra I think would be too far for a day trip. You at least need 1 night there (if not 2).
I would also recommend you do a wine tour in Tuscany. We did a wine tasting at Castello di Verrazzano (http://www.verrazzano.com/). It was only a 30 min bus ride from Florence to the Chianti Region and was near the town of Greve. It was the highlight of our trip.
It was an approx. 1 hour tour of the grounds/wine making and then we did the lunch tour option (the Chianti tradition) which was fabulous. We sat on the terrace overlooking all of tuscany which was beautiful. Each person got a huge plate of a mix of meats and cheeses (there was a vegetarian version available). Then we sampled 4 wines, and after they poured they left the remaining bottles for the table to finish (and there were only 6 of us at each table). We also sampled a dessert wine and balsamic vinaigrette. And there was also bread with olive oil drizzled over it.
I would highly recommend this place. The views were amazing and the people were so friendly!
Have a great trip!
