Ciao,
I wouldnt recomment Florence in summer for a honeymoon either - really hot, humid, heaving with tourists - cannot think of anywhere less romantic. (The highlight was an overweight tourist covered in cameras taking off his sneakers and socks inside the Duomo to massage his feet - ugh!!) Worth a day-trip though, if you are staying in the surrounding area.
However, the area around Florence is fabulous - lovely rustic villages, amazing hilly scenery covered in olive groves, restaurants, chianti etc.
As mentioned, San Gimignano and Sienna are lovely - just like how you imagine Tuscany.
Without a car it will be hard to get around, but I have to say I found the public transport great - fast, cheap trains.
Have a lovely time!