Unfortunately, you are not going to have much time to enjoy Venice. A day or two extra is preferable. Hotels in Venice proper are pretty expensive but those on the mainland are not bad price wise, however you have to take the train or other means to get into town. We have found that quite a bit of walking is necessary there, even if one takes full advantage of the water taxis, not cheap. We use the guided shore excursions offered by the cruise line. One of our visits there involved a coach tour and no ship. My wife has arthritis in one knee which limits the amount and speed of walking she can accomplish so we are very careful in selecting shore itineraries. Even getting to the market from a ship at Marittima takes a bit of walking.
Most ships use the Marittima piers, closer yet to downtown is the San Basilica pier for smaller vessels. Our upcoming cruise in November is on the Pacific Princess and the same ship we took on a similar cruise two years ago, a small vessel, but they use the Marittima.
Our favorites so far have been gondola rides, strolling along the Grand Canal and visiting the St. Mark's Square with its Basilica, visiting the "Bridge of Sighs", taking a tour out to a glass factory on one of the outer islands, our most favorite was a night gondola ride we took two years ago, beginning with a wine and cheese treat. One tip about day gondola rides, check the tide levels, since the island is sinking it is difficult to maneuver the gondolas under the many bridges in the picturesque canals at high tide, but they take you anyway, and your money, though you spend most of the gondola tour out in the Grand Canal.