I would recommend that as soon as you arrive, you make a reservation for Xmas eve dinner. You'll probably be limited to some better / tourist-driven / hotel options, but if you don't do this, you may end up eating snacks out of packages from a kiosco.

Xmas day if the weather cooperates (i.e., no nasty thunderstorms) will be your perfect day to take a lovely walk and do your picture-taking. By the afternoon, there should be a few places opening up to eat. And, you may stumble on one or two places open earlier in the day. If you are staying at a hotel, have a nice big late breakfast.