Buongiorno,
I suggest that you base yourself for two nights in Florence - you get in late, just head over to your hotel, check in and either crash or go out for dinner.
Enjoy one full day, a second night and then pick up your car later in the afternoon and drive to your next accommodation.
You can base yourself in Siena - though for your price range you might enjoy a b&b outside of Chianti - one with great views, and a bit of romance. From that base you can enjoy your vineyards, your olive groves, and visits to many of the major sites in Tuscany.
Have a look at these places:
http://www.tuscanyaccommodation.com/...el-giglio.html
http://www.tuscanyaccommodation.com/...malaspina.html
http://www.tuscanyaccommodation.com/borgo-argenina.html
or a self catering apartment:
http://www.tuscanyaccommodation.com/...a-vacanze.html
Buon Viaggio,
Donna Denise
PS - these links will have you booking direct with the owners for the best prices
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