I highly recommend only doing Saint Marks Square and Rialto Bridge on one day. Its easy to get stuck in those places all the time....they are worth a visit once during the trip.

Spend sometime on the Island of Murano, take a tour of a Glass Blowing Factory.

Our favorite thing to do, is take the Boat to the outer islands of Burano and Torcello. There is a 13 century church, and far less crowds.

We typically get an AirBnB in the residential sections, and spend time just getting lost in the City. When the kids were little, we would Grab a Pizza, find a super small bridge in a no traffic area and eat. Magical.

A must, take a Vaporetto ride down the Grand Canal at night, and try to sit in the back seats.

We have been 3 times, and have spent over 20 days in Venice proper. I would love to share my experience in BM.