I'd definitely second the secret tour of the Doges Palace - really interesting, creeping about in the attics of the palace, the Bridge of Sighs, cells, torture methods, the story of Casanova, looking down into the areas where tourists were below us. It was really good, and I would think teens would love it. Book it as soon as you can, tickets are limited.
Also, if you happen to be there on a Saturday, and you like fish, go to the Muro Vino e Cucina, by the Rialto Market at midday, they run an outdoor kitchen serving Fritto Misto, and a glass of wine, for 10 euro, then you just sit or stand in the square eating - really great atmosphere.
Venice is also famous for cicchetti, small, tapas-like snacks, which you can find in many bars and cafes, especially in the early evening. They make a great way to try local foods at a couple of euros a plate.