DD's stocking always has some sweets/chocolate, and half a fruit bowl (red apple, green apple, the greenest banana Santa can find (DD favourite kind!), mandarin, and something like a peach/kiwi/nectarine). (The fruit harks back to my childhood, when having this fruit, ALL TO YOURSELF, was a major treat in our house - generally we had a large box of golden delicious apples to share so a Granny Smiths (my fav) and all the other types of fruit were a real pleasure!).
Santa always brings at least 1 book - a reading book that he thinks DD would enjoy, and sometimes 2 or a relevant factual book or joke book as well.
There is always something useful - generally nicer than the useful things Mum and Dad would normally buy - so coloured underwear (instead of plain white), socks, hair bobbins, magic flannel, - things like that.
And then a few fun things - which can vary a lot in how many and what over the years. We've had things like craft materials and kits, small jigsaws, little toys (different things like spinning tops, toy race cars, slinky, marbles, those animals that fall down on their elastic when you push up the base, mini-figures, collector cards, small soft toy, playdough, aliens in an egg of slime (no WAY would Mum allow that in the house!), extra furniture for the doll's house or outfit for a doll, McD's or similar kid's meal toys, all sorts).
DD's stocking is roughly 12" tall, so doesn't fit a huge amount into it. But they are a few different types of things that have been in it on each of the past 10 years.
So far this year, I have a mini lego set and a mug with the sailing flags on it (she likes sailing and wants to learn them), I have seen an Oreo pencil case (with a pack of oreos) that I will pick up on my next trip, and she will probably need a new set of bobbins. I will get the fruit at the last minute, and I haven't yet seen a book that she'd love. But there's plenty of time to pick up maybe 2/3 more items and the book.