You could visit the Murano and Burano islands that are a ferry ride away. Within Venice you should visit the Rialto market and the bridge. Take the hour long ferry circumnavigating the city, in the evenin g, for the sunset views. Besides St Mark's basilica, you could visit the Doge's palace and Correr museum, both with an entry fee. Take an evening 4 pm free walking tour to learn more about the history of Venice and Venetian masks. Last, but not the least, just walk around until you get lost in the city. Trust me, you'll find your way back :)