St Mark's Basilica in Venice is the city's most famous and likely most crowded church, an excellent example of Italo-Byzantine architecture. It is not, however, Venice's most attractive or most interesting church on the interior, compared with some of the later Renaissance and Baroque styles.
What is truly lovely and unusual at St Mark's, however, is the treasury—particularly the half of the treasury featuring dozens of gold and crystal reliquaries containing the bones of saints, all arranged in orderly rows and carefully labelled as if in some demented science museum.
The treasury is a pleasant break from the cattle call of tourists doing a quick loop through the main nave, and is well worth the extra five euros to get in.