I tend to buy very much what we'll eat as a family over Xmas plus for visitors if they're expected. I also buy some extras for unexpected guests.
However, the last two years we've had no one unexpectedly and we've been left with meat things, mincepies, salad stuff that's gone off and having to eat party packs we don't really fancy. So would you buy a few things in just in case? If so, what? I'm thinking along the lines of things that keep. Freezer is full of our normal food.
If someone caught us on a good day, we'd have a couple of packet of crisps, a couple of cheeses, can tuna, hopefully enough bread, a dip, some salad packet of mince pies (no meat), but that will get used over a few days.
None of us drink fizzy drinks, nor do our expected visitors. Again, would you buy them just in case? We'd only have squash, water or wine on offer.