The Vatican Museum holds a huge percentage of the world's fine arts, and of course everyone goes to visit the Sistine Chapel. On your way, just before you reach the chapel, you'll be pushed by throngs of people through the hall of maps. I love maps, and these are not only fascinating in their detail, but in their accuracy as well, considering they were commissioned in the late 1500s. The Sistine Chapel was great, crowded, and amusing considering the master who stands in front of the room shooshing people and announcing that pictures aren't allowed, which hardly makes a dent in the brilliance of a thousand flashes going off each minute.
Here is a map of Venice. Click next and you'll zoom into the close-up picture of San Marco Square.