I'm in the Venice is great crowd (but I'm the kind of person that likes everything).
St Mark's is pretty amazing. Entrance to the basilica alone is 6 euro. Twenty euros gets you admission to the basilica but also allows you to see the Pala D'Oro (the enormous gold altar piece which was far more impressive than I expected it to be) and the Museum (which includes the original Triumphal Quadriga [the horses of St Mark's], which were cast in antiquity, stolen by Napoleon, and returned in 1815). The Museum ticket lets you walk out on the front roof of St Mark's, too, which offers a great look over St Mark's Square. My friend and I got the 20 euro ticket; we were both glad we did. Pre-purchased tickets allow for "skip the line" admission; you definitely want that -- the line for non-ticket holders to get in is CRAZY. I would strongly suggest going as early as you can in the morning. That area gets mobbed during the day. We did St Mark's tower first at opening time (not a "must," to me), and then headed over to the basilica. We were allowed into the basilica close to an hour early (my friend thought the tower would take longer than it did).
Doge's Palace is interesting, too. You will see a "Secret Itineraries Tour" on the website. It explores the non-grand administrative side of the palace. That tour definitely is not a "must," and I think I would probably skip it with the time you have.
Catch a vaporetto from St Mark's Square over to San Giorgio Maggiore for its bell tower. The tower visit is something like 6 or 8 euro. Not as tall as St Mark’s Tower, but the view toward St Mark’s Square and the far smaller number of tourists makes it a superior experience to St Mark’s Bell Tower, in my opinion.
Dueling orchestras on St Mark's Square in the evening. I think this is a ton of fun, but it's pricey. Hot chocolate 14 euro + 6 euro "music fee" if sitting outdoors. Or you can just walk around on the square and enjoy the music.
Then get away from the center. The old Jewish ghetto (non-WWII use of the word) is interesting to explore and there is a 60-90 minute tour of the ghetto, including synagogue visits.
Maybe interest in visiting a marionette workshop? L’isola di Pinnochio.
And, as mentioned above, wander wander wander as far from St Mark's and the Rialto area as possible.