I think that a day and half will give you the barest taste of Venice. Long enough to make you realise that it was not long enough, and you'll maybe be planning your next visit before you even leave. In a day and a half, you can maybe see the big ticket sights, San Marco, Doges Palace, maybe another thing or two, but you won't really see Venice. It's good to see how Venice really works, the insanity of a major civil, economic and maritime power, built on a bunch of mud islands. You can't see that side of venice in such a short time.

Try and give yourself an extra day at least. I should confess - I'm a bit of a Venice enthusiast.

At the very least, take the vaporetto to San Giorgio stop. You will be in front of the San Giorgio Maggiore church. Go inside and toward the back of the church. Follow the signs to the belltower.

Take the elevator up. You will have the grandest views of Venice available today. You should do this during daylight hours. I promise you will not regret it. :)