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You really should spend your time on whatever catches your own interest.
Some people would rather get a root canal than spend several hours in the Accademia, but for others an afternoon of Old Masters would rank as the highlight of their trip.
Same goes for shopping, or gondola rides, or cramming a dozen churches and museums into a single day: Heaven for some, hell on earth for others.
For some less-famous sights to visit, check out Reid's List: Venice.
Keep in mind that you may have to adjust these itineraries in case one of the days you're in town happens to fall on a Monday (when most museums are closed) or a Sunday (when many things are closed, and those that remain open tend to operate on shorter hours).
If any of your days in Venice happens to be a Sunday, do not miss the 6:345pm mass in the Cathedral of St. Mark's—the only time they throw on all the light switches to illuminate all of those amazing gold mosaics.
Keep in mind that these are maximal itineraries, designed to cram as much a reasonably possible into the time allotted.
(Though you will notice I pad about 10–20 minutes between stops to give you time to get from place to place. That's why if, say, it says to arrive at Sight #1 at 9am and spend 45 minutes there, Sight #2 doesn't have you arriving until 10am—that "missing" 15 minutes is walking time.)
Aside from that, there is no down-time built in for relaxation—which you really should have.
You're on vacation, after all.
I suggest using these but maybe dropping a sight which interests you less (or curtailing your time at a couple of sights) in order to carve out from some free time to just sit at a cafe, writing postcards and watching the carnival of Italian life swirl past.
This is all meant to be ballpark. What is currently open in town (and those open hours) changes more frequently than you might think, so always make sure you get updated open hours and other intel at the Venice tourism information site:
www.turismovenezia.it
