Third day:
Maybe have a tour of the Doge's palace (is it worth 25 euros each?)
I don't think the Doge's Palace was that expensive. I think perhaps that price is for the 'Secret Itinerary' tour. Or perhaps it's for the audio guide? (Which we also didn't get, but is probably useful).
I didn't do it myself, but the tour takes you to places that you can't see just wandering about yourself, such as Cassanova's Prison Cell.