You may want to bring some lightweight clothing like a dress, or a nice shirt with shorts. Keep in mind some places you visit in Florence (like the Florence Duomo) have a dress code. Typically, the dress code is to make sure you have your knees and shoulders covered.

Florence does not have a city-wide dress code for walking the streets, but strict rules apply when entering religious sites like the Duomo (Santa Maria del Fiore), Santa Croce, or Santa Maria Novella. To enter these cathedrals, both men and women must have their shoulders and knees covered. Tank tops, sleeveless shirts, short skirts, and shorts above the knee are prohibited. It is common to see tourists carrying a light scarf or pashmina to wrap around their shoulders or waist if they are wearing summer attire. Additionally, hats and sunglasses must be removed before entering, and footwear should be modest; while sandals are generally allowed, flip-flops are often discouraged. In high-end restaurants and theaters, "smart casual" is the expected norm, and Italians generally dress quite formally compared to North American standards, so avoid "athleisure" if you want to blend in with the locals.