thanks for your message.
There are lots of interesting destinations you can choose from for a day trip from Florence!
Of course Siena, Chianti and San Gimignano are all highly recommended, though you’ll not be able to fit all of them in one day. How about your way of travel? I imagine you’re using public transportation for the day tour from Florence, on the contrary please advise.
With the use of train and or bus from Florence, I suggest you visit San Gimignano and Siena in one day. On the blog I make a list of the top day trips from Florence, that also include other great destinations like Pisa and Lucca. Take a look and see what can most inspire you.
Please get back to me if you need any further advice,
Ciao from Florence!