Thanks very much to all,

For now we will stick to our intial plan. And since lodging is more expensive in Venice 3 nights (2 full days) will be for that trip. Enough or not we will see. My idea idea is that we,re still lucky to have 2 days in Venice. The large part of the people around me wont ever see it.

We have found an appartment in Cannaregio. Can someone tell me if an appartment near campiello l'anconeta will be fine for restaurant and walks ?

Yes. There are some good places there and also a five minute walk away. La Columbina is near, and it is very good though a little pricey. There is also the coolest little bar called Ostinati for cichetti and wine, hidden away in a little courtyard. It has a sweet little garden.
https://www.facebook.com/Ostinativenezia/
If you do your homework you can eat extremely well in that neighborhood.

To get to a lot of sights you will need to take the vaporetto though.