The Basilica di Santa Maria del Fiore (here in LEGO form by Riversari) is the jewel of Florence. It is often simply called the Duomo, which means cathedral or church. The word is derived from the same root as domicile, since this is the 'house of God', but in this case is particularly fitting because it also sounds like 'dome', and Brunelleschi's amazing dome is certainly one of the structural highlights. I've had the pleasure of visiting Florence several years ago, and remember how awe-inspiring it was to be inside this. Also, you can climb up the Campanile (bell tower) built by Giotto, and get a great exterior view.