Florence is world renowned for its Renaissance art, including paintings and sculptures from the most famous artists of the time including Michelangelo, Brunelleschi, Leonardo da Vinci and many many more! This is, of course, the allure of coming to Florence to see the majority of the Renaissance art and to hear the stories of the conniving families and dramas that existed at that time, Florence is full of secrets and mysteries.
The must see spots for culture is the Accademia museum where you can see the real David of Michelangelo (there are several throughout town so don't be fooled by immitations!) as well as other paintings and sculptures from lesser known Renaissance artists. The Uffizzi which is four floors and has the most famous paintings such as Botticelli's "The birth of Venus" and works by Raphael, Leonardo da Vinci and more............
Obviously, you must see the Duomo cathedral, a main attraction in Florence however with the lines being up to several hours long in high season you may want to skip it and climb the tower instead if you are not afraid of tight spaces and heights. I must warn you there is no turning back once you begin, you will have a full stairwell of people behind you so if you have a weak heart I suggest you visit one of the rooftop bars (I can give you suggestion) and enjoy the view with a nice spritz at sunset, that's my personal preference:)
In addition to that would be the Pitti Palace in Palazzo Pitti where the famous Medici family once resided. You can enjoy the 12 salons of the Medici where the family lived and they are quite elaborate and of course the beautiful geometric gardens with another nice view of Florence, you can even bring a panini and a bottle of wine and have a picnic. Of course, the best tour guide in town is Aniko of "Florence and Chianti tours" if you want to enter further into the history and intrigue, she is so fun and entertaining;)
When you think of fashion you may think of Milan, seeing that it is up there with Paris and New York in regards to being one of the fashion hot spots internationally, of course, hosting its own fashion week as well.
However, Florence has some great museums that you can visit to get some of your fashion fix if you cannot make it to Milan. As well as some great shopping, if you want to know the hot spots, were the local fashionista's go you must contact Jana owner of "shop in Florence" and fellow colleague at Salvatore Ferragamo, she will have you looking like a super chic European in a day!