Based on the few days I spent there in 2002, you can't miss. Everywhere you look there are amazing examples of art and architecture. Some buildings are over a thousand years old (San Marco was built around 831 IIRC) which is quite a novelty to someone living in the US where "old" is typically from the 18th century. And as ntenny notes, the city is quite walkable, it really isn't that large (although a few of the streets are pretty wet ). Unfortunately some museum and church interiors are off limits for photography.