I guess I don't understand why you would sleep in Siena and day-trip to Florence. I've spent weeks in Florence and there's so much to do, so many sites. I took a day trip to Siena, and felt a day was enough. I intended to return again and spend days in Siena, but after being there for a day, felt I'd seen it. This is just MY opinion. (also to her surprise, my best friend had the same opinion) To me, it was a lovely small town, with one pretty piazza, but I just didn't "get it."
Florence, on the other hand, is a transportation hub, and there are many day-trips you can do from there, including Siena by bus. You can also do wine tours, etc., from there, and pretty much see all of Tuscany without a car. If you have a car, you may enjoy staying in the countryside and day-tripping into Siena and Florence, but Florence by itself deserves three whole days, IMHO.
Siena is pretty, but to me it was on the same level with the Spanish Steps in Rome, that I'd longed to see for years and years, and sang the song, "that's all there is?"
Just me.