We were in Venice Dec 24-26 many years ago. We wanted to ride in a gondola before going to St. Marks for a vespers concert. We got lucky and found one gondola operating. We didn’t see another gondola the whole time, even on the grand canal. We felt like celebrities, with everyone taking pictures of us. The vespers concert was beautiful. Shops will probably close early on the 24th. At midnight, the churches ring their bells.
On Christmas, we went to Murano. Most shops were closed, but we found a couple that were open. This might be a good day to take the trip to Murano and Burano and just explore.
I agree that your hotel is probably your best resource for restaurants that will be open. We just returned from France and Spain and found many restaurants closed during hours that were listed as “open” on their website or TripAdvisor. Many restaurants were closed for the season.