I wrote a long response on your other thread. Now that I see you will be there for 2 weeks (lucky you) I will reinforce my suggestion to get a good map and combine that with whatever app you choose. It is the best way to learn your way around Venice like a local.
I recommended booking a visit to the rooftop terrace of the Fondaco dei Tedeschi, and provided a link. Do that ASAP,
I will also recommend the Saturday tour of Ca’Foscari, the university headquarters. A nice free 40 minute tour with a student guide is offered on Saturday mornings at 10 am, 10:40, and 11:20. We went yesterday and really enjoyed it.
You do not need to book; just go into the courtyard and look for the little tour desk the guides set up there.