I agree with pretty much everything said above. Venice is a very walkable city with a lot of quiet little corners; you can just pick a nice-looking square next to a little canal and wait around for the light to do something nice. Piazza San Marco really is very impressive in spite of its tourist credentials, but the real charm of the city is to be found elsewhere IMHO. I don't remember getting much use out of a wide angle on my trip there, but I agree with Darko that "tight and narrow" is a common theme of the place.
You may want to take something handheld as well, unless you have your LF rig well set up for extemporaneous walking-around shooting expeditions.
If you ask around you may be able to find a boat to take you to the abandoned island of Poveglia; they're not supposed to, but rumor is that it occasionally happens anyway.