Freco I usually put a christmas comic in too - they often come with something attached to the front as an extra little Bit of tat but also take time to read and look through and do the puzzles etc. And depending on the size of the stockibg it makes a useful cheap space filler and gives the leg something to keep it straight. If he's too young for a comic then some sort of activity book that they will want to look at.
Nothing with any hint of messiness about it - no scooby character in goo (voice of experience on that one!) or playdoh or glitter etc.
Small xmas craft kit that's not messy and they coukd attempt themselves.
Small hotwheels type car - usually a pound in poundland or less for a non branded one.
Any food or choc - make sure yoy don't mind them eating it before breakfast and that they can open it without creating mess
Vest, pants, socks that you want them to wear that day - makes getting dressed easy in the morning.
Soft toy poking out of the top - the kinder plushes are good as they come with choc too and its exciting to see a new little friend looking at you.
I put an asda soft micro plush christmas throw with all stockings a couple of years ago (adulrs and kids!) which went down very well - they're £5-6 this year and very cute and soft.
Wilko has cheap little lego kits of a good size for stockings for a pound or two - their own brand but lego compatible. And again good for takibg time to be done.
Anything tgat has packets that you want tbem to open make sure you make it as easy as possible so they dont need you to do it there and then.
A christmas dvd to watch then or a bit later when you need to be cooking lunch or whatever and want to have something calm to distract them. Can highly recommend tom and jerry a nutcracker tale if you dont have it for something easy to have on in the background