For Christmas day evening I second smoked mackerel pate, on the grounds that you'll have had enough meat for the day and it's utterly delicious and really easy. It's just smoked mackerel, cream cheese, lemon and black pepper whizzed up in the mixer, or with a fork. Also maybe a large amount of one single cheese - stilton would be appropriate but I'd probably go for cheddar - because one big cheese always looks more generous than lots of small bits. Serve with oatcakes (you can get mini ones which are cute), butter, and a huge bowl of crudites, incl celery sticks (you can get rid of the stringy ridges with a veg peeler, which makes them much nicer to eat), carrot, peppers, and raw cauliflower florets are good too. For the cheese, you'll need some really good chutney or pickle.
Some good posh salads:
- beetroot, goats cheese and walnuts on some soft salad leaves (use walnut oil for the dressing if you want one)
- warm boiled peeled potatoes tossed with matchsticks of smoked streaky bacon and warm vinaigrette; or warm tiny new potatoes roasted with lots of garlic and rosemary
- roast root veg (which can be cold, ie left over from earlier) with puy lentils (done the day before), watercress, crumbled feta cheese
- chickpeas, fried chorizo, cherry tomatoes, minced red onion, flat-leafed parsley (sherry vinegar dressing, balsamic if that's easier). If you don't want meat you could just do the tomatoes and onion.
For Boxing day, I'd do cold meats (leftovers) and a gratin as TMOC suggests, but maybe of celeriac for something different? Bear in mind that will be lots of white food, so you'll need to serve something else - maybe a watercress salad?
OR you could chuck all your leftovers into a pie: fry some leeks slowly in butter then make a white sauce around them with flour, then warm milk. Then chuck in basically everything you've left over from the day before, including roast potatoes, topping up with any gravy, stock or white wine you have knocking around so it's nice and moist. Spread into pie dishes or anything that's ovenproof, and cover with ready-made puff pastry. It'll take an hour in total to prepare and cook. Serve with baked potatoes and lots of steamed fresh green veg. This is the traditional Ball household Boxing Day supper and it is delicious.
Sounds fun, enjoy.