mine typically get: (2x ds, 7&4 this year)
chocolate coins and satsuma at the bottom
New vest, pants, socks and t-shirt (all ready to wear on christmas day to lessen trauma of getting dressed rather than running straight downstairs)
new toothbrush and flannel. new comb (if yours are anything like mine and keep losing them). any other little bitty things that are quite boring but they need on a regular basis that you would buy for them anyway.
small packet of lego - the lego shop does a good one most years that is christmas themed, in a bag rather than a box and costs about £3. This year they had a good santa and sleigh, and a less good snowman and tree I think ( got the santa one so don't remember the other).
Maybe a small packet with a mini figure in - either lego, playmobil or similar, whatever they like.
toy car or two (or maybe a car and bike or plane etc) - depending on who has the best deals on hotwheel type cars.
card game - both my dc love these. either top trumps or flash cards or snap type cards with Cars or Mickey Mouse or Peppa Pig etc on. again, lots in the supermarkets and toy shops for a couple of pounds each at the moment.
small pack of crayons or felt tips and small colouring book or notebook - supermarkets do nice little sets of pens for less than a pound. poundland has good selection of colouring books and notebooks.
christmas issue of a nice comic with stickers in and plastic tat a toy on the front to give them something to do
If you suspect that there is likely to be time where you want them occupied in the morning before the main christmas presents are opened and/or before guests arrive etc - then a dvd so you can stick it on to keep them busy while you get on and make the lunch or whatever it is you need to do!
Anything else like that - think through christmas morning/afternoon and think if there is anything that you can stick in the stocking to make your life easier for the rest of the day 
Also...
avoid anything that has any mess in a bedroom potential, especially if the recipients are likely to a) open them before you are awake or b) open them in your bedroom on your bed. Things like playdoh or scooby doo scream gel or even toothpaste or pearls of bubble bath.