You're in for a treat! I travel cross-country just to see performances at the Met, in fact, I was just there last weekend for "Lulu". It's one if the greatest opera houses in the world!
As mentioned by others, try to get there early and soak in the ambience. There's always interesting exhibition at the lowest level. Yes, unfortunately the chandelier pre-show thing isn't done this season.
For intermission, yes, get the (overpriced) champagne and mingle with the elites
I actually quite like their sandwiches too. My favorite is the grilled chicken on croissant.
A lot of people recommended the restaurant on the grand tier level, although I never tried it myself. I think the concession at the grand tier level sells coffee, but the one on orchestra level doesn't.
Anyway, have a great time!! I'll be back there in January myself! :)