The terrace of Rinascente offers a spectacular viewpoint over the Basilica di Santa Maria del Fiore. In the terrace is also possible to grab something to eat or drink.
It's quite a fancy restaurant as it's part of “Rinascente”, a luxury shopping centre located in many cities across Italy, sort like Galleries Lafayette in Paris or Harrods in London. If luxury shopping is what you're after though, save this spot also for that :)