I love cheese and fruit so you'd be providing the best bit for me!
Fruit: apples, bananas, satsumas, grapes, pears (if you can be sure they'll be just ripe but not over-ripe - achieving this is a minor miracle, in my experience!), maybe dates, figs - you could also get some prepared fruit, e.g. fingers of pineapple, mango and melon. If I was attending a Christmas buffet I'd really appreciate some fresh fruit to offset the vast amount of carbs I'd be consuming.
Cheese:
Good strong Cheddar - extra mature for me
One out of Wensleydale, Lancashire or Cheshire - in other words, a crumbly British cheese - a farmhouse version will be better than the bog standard supermarket version
Wensleydale with cranberries - the purists will hate this but I really like it once in a way, and there is so much of it about at Christmastime I have to assume I'm not alone
Maybe another novelty cheese, such as one with chives, mustard, chilli or whisky in it - again, lots of those about at Christmas
Brie or Camembert
Gruyere, Emmental, Edam or Gouda - i.e. a continental cheese that contrasts with Brie/Camembert
Something blue - I'm not keen on blue cheese but Stilton is traditional at Christmas. Dolcelatte is less strong and much creamier.
Crackers: easiest way is to buy some of the selection boxes so you get a range of shapes, flavours and textures. I love Carr's water biscuits with cheese so would always make sure I had some of those. Those little Hovis digestive biscuits are very good with Wensleydale etc. Oatcakes are essential for me too.
Chutneys - a few contrasting types would be nice.
Celery sticks? Good with blue cheese.