A good idea is to take a tour throughout the center of Florence in Summer evenings, when the city is full of lights and offers its best nightlife. The focal point is, without a doubt, the area between Piazza Stazione and Piazza del Duomo as far as Ponte Vecchio. The busiest places are:  Piazza del Duomo, Via Roma, Piazza della Repubblica, Via Por Santa Maria, Via dei Calzaiuoli, and Ponte Vecchio.  It is very hard to reach this area by car at night, because car parking is impossible to find,  so the best choice is to get there by bus, by taxi or better walking along the romantic Lungarni (River Arno banks) illuminated by the typical Florentine streetlamps reflecting on the water and creating a magical atmosphere.

Unfortunately Boboli gardens close at sunset but most active people can reach the panoramic Piazzale Michelangelo, walking up through the beautiful 19th C flight of stairs among flower-beds and fountains. Next to Piazzale Michelangelo emerges the imposing Fortress of "Forte di Belvedere" with a gorgeous view of Florence and an enjoyable gentle breeze to refresh visitors.
Even  higher up the soaring Renaissance Church of St. Miniato with its lively green-and-white marble facade and the beautiful staircase is a very romantic place worthy of a visit.