Florence is famously cliqueish so don't plan on getting into interesting social circles on your vacation. The city does have a few bourgeois divertissements to offer, though.
When I was in Florence, Un Ballo in Maschera was being staged by the local chorus and orchestra with singers from around the world. The opera was ostensibly set in Boston, but the set designer couldn't resist throwing Roman columns into the "deserted field" scene. What was most Italian about the experience, however, was the divided reception the lead tenor's arias received. About one quarter of the audience would clap wildly and one eighth would boo fiercely. It seems that there were two tenors engaged for this run and fans of the singer who had the night off were booing.
Opera in Italy is a bit different from American performance. The orchestra pit is deeper and the orchestra plays softer. Opera is for the singers here. As in New York, they've raised the prices so high that getting a seat at the last minute isn't all that tough (I paid 120,000 lire and sat 7th row center). Don't expect the kind of Baroque splendor you see when James Bond goes to the opera in Vienna; the Teatro Comunale is a fairly new and plain building, remodeled after the 1966 flood.
There are allegedly two theaters in Florence where you can see movies in English on at least one or two days each week. I was there the week before Christmas and both were closed until sometime in January.
"You really have to go to the riverfront park just on the west side of town [Le Cascine?]," Barbara, an American artist living in Florence, told me, "because every night the most extraordinary smorgasbord of women assembles there. You get 16-year-olds from the Balkans and Russia and they really dress the part. The stand on the street and traffic moves by at about 2 mph. I've always wanted to stand on the sidewalk there and sketch, but I'm afraid the pimps will chase me away."
I didn't learn this until hours before my train back to Rome, otherwise I would have asked a cab driver to "take me to the puttane" and braved the pimps to get a few snapshots for you.
I spend most of my evenings writing so I'm not really qualified to describe what the restless should do at night in Florence. Perhaps one of you gentle readers will contribute a report.... Just send me email or add it via my comment server (below).