In the summer, especially on weekends, join the Florentines for a stroll in the Piazza della Signoria or Repubblica or down Via Calzaioli after dinner. Tourists and locals alike stay out well into the night, sometimes as late as one or two in the morning. Street performers of the moment include human statues that move only when someone drops lira into their hat or cup, fire-eaters and twirlers, singers and slapstick comedians and can be found mostly near Piazza della Repubblica.
Didn’t get all of your souveneir shopping done during the day? There are posters of famous Italian paintings or cityscapes, African art, fake Gucci bags and spray paint posters to buy.
Occasionally there are outdoor concerts in the Uffizzi walkway or Piazza della Signoria. Landmark caffes are open late, gelaterias aren’t.
Don’t walk around areas you aren’t familiar with by yourself and definitely avoid the Cascine Park at night, especially if you are a woman traveling alone.