We found a couple of nice bike shops in Siena and we rented bikes from one that was just outside the walled city. The owner speaks English and brought us and the bikes to the beginning of a route he planned for us (OK, we went off the route and climbed, oh, about 2K ft. up a mountain pass), just outside the horrendous traffic in the city. Siena is a quick train ride from Florence and really not to be missed. Of course, I am prejudiced, as my son lived there for 6 months, but if you are in Florence, you should do at least one day there.
And, I agree with Mr. Bloom; don't over plan.