Florence and Siena are beautiful, I enjoyed my time there back in early 2009. Hopefully they will have some free time thrown in to check out the city as, I found, wandering around some of the side streets and kind of getting lost was one of the best experiences of my time in Italy. Plus, nowadays, there are all sorts of apps (offline maps, etc.) that are user friendly and will get you right back on track as soon as you have become fed up wandering. Any way, I digress, sounds like an awesome trip. I have not been to Athens yet but I have heard it is fantastic!