Firenze Duomo
Gush as I may, the churches and cathedrals of Europe are like no other.  This Florentine beauty is made out of three different marbles- white, green and red- the colors of the Italian flag, by the way.  This stunning dome lets you access it from the inside, outside, the dome roof (closed when I was there) and the bell tower, or Campanile.  The marble work is first class, and the patterns will keep your eyes busy.