A few "not so obvious" things to do in Venice:
Walking tour of the Jewish Quarter. Do a little reading about it beforehand, or take a guidebook that includes walking tour highlights.
Museo di Palazzo Mocenigo. I didn't think I'd enjoy it, but this was fantastic. The museum contains collections of rare Italian fashion and costumes, mainly from the Correr, Guggenheim, and Cini collections. They also have a really interesting collection of perfume bottles going back centuries. If you're into design, it's well worth the time. https://mocenigo.visitmuve.it/en/home/
Ca' Pesaro Galleria Internazionale d'Arte Moderna. The Peggy Guggenheim is definitely the first stop for art in Venice, but if you have time for a second museum, this one is very good. https://capesaro.visitmuve.it/en/home/
Those two museums are within a 2 minute walk of each other.
I agree with Irwin that a side trip to Murano is worthwhile.