St. Mark’s square:
Procuratie Vecchie e Nuove, Napoleonic wing, Clock tower, Bell tower, Biblioteca, Doge’s Palace with State Prisons, Bridge of Sighs.

This tour is a must if you visit Venice for the first time or if you haven’t long, but it’s also the base to fully comprehend the glorious history of the Venetian Republic. (duration 2h)