I know a few people have mentioned stocking up in the weekly shop - very useful to do. I also do this, but on weeks where I don't see what I wanted, I buy extra supermarket savings stamps instead. With the idea that come the "big" shopping weeks for perishables etc, and when all the BOGOFs and 50% extra free deals are on for "Christmas season staples" (eg. Hellman's Mayo, Uncle Ben's sauces for leftover turkey curry, ....) - that are things I would use anyway into the spring, I don't blow the budget stocking up on those also because I have my stamps. (And the supermarket I save for has a card that you fill €98 but they add the final stamp to €100, so a bonus €2 extra if you fill it).
I try and have a range of meals in the freezer for the busy weeks of December - when we are racing in from work and back out to school events etc. And I try to keep them in either 1 person portions or decently large family sizes. So I use plastic takeaway tubs for sauces (can write the contents/date in sharpie on lid and it easily washes off again). And I have a few pyrex glass dishes for lasagnas etc. The Pyrex dishes can be reused by emptying the contents once frozen and wrapping in cling film/tin foil and labelling, and just unwrapping to pop back into a clean pyrex to defrost and cook - so I don't need loads spare. And the 1 person portions mean that it's easy to multiply a meal if more people are home for dinner, or we get visitors, by just taking out however many more we need. And 1 person portions defrost quicker (especially useful if you've forgotten to take out of the freezer the night before...).
I have to send a lot of cards, and quite a few abroad. In normal times, I tend to have a lot of travel, so I bring a pack or 2 with me to write on the plane (within EU so not long enough for a movie and I give enough time to work on those trips so don't want to work on the flight). And if not, I try and do a batch of 10 each week for a few weeks, which only takes a few minutes each - so reduces the panic when final posting dates loom (especially for far flung places).
I also keep a box (small shoebox cunningly disguised by both box and lid being separately wrapped in christmas paper) under the tree - with emergency tools - a roll of tape, a few cable ties, spare bulbs for lights, screwdriver, small swiss army knife, 2 bin bags folded neatly etc - useful for if something needs fixing quickly, but also when opening presents, putting away paper, cutting zip ties or opening tricky packaging, putting together something etc.
For a nice breakfast on Christmas morning, I put a pack of M&S pastries in the freezer in advance, to bake when we wake. (I sometimes remember to set the oven to heat up on the timer in advance, but not always).
DD has always made cookies on 24th for Santa's snack. We use the same recipe every year and we always make at least 1 batch earlier in December (they are nice to eat generally). I freeze half of the earlier batch. So on 24th, while we are all working together on peeling veg and making stuffing etc, preparing for next day, DD will make the cookies from scratch if we have the time/energy to do that. But if things go pear-shaped for any reason (run out of a vital ingredient, too busy, people calling so we need to stop early, DD having a meltdown for any reason (ADHD/ASD), there can be 101 reasons why things go pear-shaped....), we have a half batch that we can just slice and bake straight from the freezer, and still have the correct snack.
I know not everyone likes them, but we do a Christmas Eve Hamper after dinner, to start to signal downtime and bedtime (important as routines are very important in our house, and sleep!). Youngest lights the Christmas Candle and we have a family prayer. Then the hamper emerges, including DD's stocking, her plastic plate and glass from toddlerhood, her snowman covered HWB, new winter PJs for everyone, hot choc for everyone (and a Christmas craft beer for DH), festive bath bombs for DD and I, and the family copy of "Twas the Night Before Christmas". DD lays out the snack for Santa and her stocking, and then goes up for a warm relaxing bath and gets into new PJs. She comes down to check the Santa tracker, enjoy her hot choc snuggled up on the couch, then goes up to bed with her HWB for the annual bedtime story and a good sleep. Which means it is a gentle unwinding and she is relaxed going to bed, so likely to sleep.