Houses the St Vitus Cathedral which in turn houses what is my favourite stained glass window. Also houses the Chapel of St. Wenceslas and the burial places of many Bohemian kings and Holy Roman Emperors. Fantastic example of Gothic architecture.
Make sure to walk over to the Ile-St. Louis which is connected to the island where Notre Dame is by a footbridge. Less tourists, better place to grab a bite or ice cream (see Berthillon) and a cool neighborhood.