There is a bike rental service organized by the city. Bikes can be hired at several points in the city (and returned to the same place). One of the most convenient for tourists is located at SMN station.
While there are hills north and south of the center of town, almost all of the historic center of Florence is easy for bikers, because it is as flat as a hat - flatter than that. But there is a problem: Traffic is terrible, and buses, trucks, cars, motorcycles, motorbikes bicycles, and pedestrians are all fighting for almost no space at all, so you'd better pay attention.
Beyond the city bikes, some of the hotels in town provide their guest with free bicycles. Bike shops also often rent bikes and some of them organize guided bike tours in the countryside.