I always make sure there is something from each of 4 categories in the stocking:
"Something you want, something you need, something to eat and something to read".
There is always at least 1 book (an age-appropriate fiction book), and sometimes there can be another 1 or 2 (another fiction or a factual book that looks interesting for that DC - on one of their interests, some inspiring people and their achievements, or "how to have fun building camps" or other nature type books).
In our house, there is both fruit and sweets to eat. I have seen others putting individual portion packs of cereal in stockings, or juice cartons. But generally, here, there are about 5 pieces of fruit:
A perfect apple
An orange and a clementine
A (green) banana (that's what is preferred by DD)
A kiwi or a pear
And then some unusual sweets (I often find these on my work trips abroad - won't happen this year) or 1 year it was a pack of oreos all to DC and presented in an Oreo pencilcase
Needs:
Things like hair bobbins, hairbands (DC is quite fussy about these), if a new tangle teaser brush is needed etc. Fancy character knickers and colourful socks that Mummy would never get.
Bibs, rattles, nappy-bag sized creams, are all good for very small DCs.
Toddlers might get a suitable sized cutlery set or plastic crockery set or sippy cup etc.
Things like hats, gloves, scarves, etc are all handy space fillers, especially if the winter is looking like being a colder one and you may need a few (cos having spares to be able to change cold wet hands, allow 1 pair to dry but send DC back out to play happily and warm again can be invaluable!!)
Toiletries sometimes end up in there too - I remember so well the year 1 DSibling got a bottle of Matey bubble bath as DM would never buy it no matter how we pleaded. DD got a bamboo toothbrush for travelling last year (she's quite eco-conscious) which went down well. And travel sized toiletries as DC get older, or bits and pieces they need to start thinking about, can be useful - a facial scrub for a young teen, or just a nice shower gel that is a kid-friendly smell not "the family bottle of radox" can be nice. Or their own facecloth with a favourite character or in a nice bright colour.
Also stationary can fall into this category - nice pencils, new colouring pencils, good markers, new eraser, nice notebooks, etc. In recent years (DD is now a teen but still gets a stocking) we've had things like coloured paper clips and bulldog clips (to hold her school notes together), fancy pushpins for her notes on noticeboard, brightly coloured stick-it notelets or coloured tabs to mark pages, and cable tidies (both stick on and magnetic to suit different needs).
Wants:
Well, all sorts. (And some of these could also be considered "needs" as in useful).
A slinky is always fun and can be useful to teach wave motion etc.
We've got various small kits over the years, a bug collecting kit, grow your own crystals, mini binoculars, etc. Some were aimed at small DCs, while others are just small sized.
Camping bits are always useful to consider too - a windup torch or radio, or battery powered versions, solar charger, compass, spork....
Or things for bikes - like spoke lights (can get fun ones) or small red and white lights to clip on, small pump, water bottle....
New balls for sports - especially tennis, hockey, cricket, hurling or squash (or those rebound balls with the bobbles on them), or a replacement for a handle (a strip of fabric with sticky on the back to wind over a worn out tennis racket handle, for example)
Gardening sets - kid sized tools and seeds to use in spring, could often be used in sandpit as well (the tools, not the seeds)
Extra bits for existing sets - sylvanian families animals or furniture, small Lego/Duplo/Playmobil sets to go with existing sets (so a Lego City Coastguard small boat or small firetruck or even minifigures to go with other Lego City things), a new set of clothes for a favourite doll or an extra piece of furniture for the doll's house, extra food or crockery/pots/tools for a shop or kitchen set, an extra dinky car to add to a collection, Schleiss animals, etc.
Small toys like little wind up things, wooden tops, skipping ropes, bouncing balls, marbles,
If DC likes the kitchen, some tools of their own - Ikea do useful colourful silicon bun moulds; proper cookie cutters; an apron to fit themselves, a chopping board (small, shaped, different coloured plastic - see what's available that is different to your own boards), small sized wooden spoon they can manage (good kitchen shops tend to have different sizes) and small bun/baking trays....and it doesn't just have to be baking, ordinary cooking too
And just fun stuff. I tended to get things that could be used up a fair amount (consumables) - like a bottle of bubbles, chalks, colours, small colouring books for travelling, sprinkles for baking, mini marshmallows for hot chocolate...
As they get older, they might use things for crafts of various sorts, and I already mentioned toiletries etc.
Decks of cards or a few extra dice often come in handy.
As they get a bit older, things to do with any hobbies they have - a sheaf of sheet music rolled up for a fun song (rather than the boring pieces they need to learn for exams), plectrums for guitars, drumsticks, rosin for violin bows, ….
Or whatever small things go with their hobby - maybe a Raspberry pi for a coder, spare (funky) laces for sports people, a pair of fun long socks for training (not just the Club/School socks for matches), water bottle, spare charging cables for tech or a power bank....
I know this is long - and DD never got ALL those things in any 1 year - just a 1 foot high stocking reasonably well filled with things from each category.
But I am just trying to give some ideas across the range from young to old(er).