Generally, annually there is at least 1 book (some are mentioned below), and also chocolate, red apple, green apple, orange, kiwi, hair bobbins, socks, knickers etc. And then:
Age 12:
"DD Name" in battery operated lights, her initial shaped post-its, Marbles, Nutcracker floppy toy, Minecraft dog plush toy, Globe stress ball, 2 reading books, small Harry Potter jigsaw, clip on light, a funny t shirt, "Lego" flexible tape,
Age 11:
HP colouring postcards, Horrid Henry fake nose, mini modelling clay, spinning top, scratching pictures set, "Inside Out" bag tag, minions cards, lush shower gel, Lego plane, putty, Match Christmas Edition, Beano christmas edition, fruit, chocolate, knickers,
Age 10:
swimmer toy, 2 bugs in glass, paint set, Coke lipbalm, lego, angry birds rubbers, wooden keyring, chess book, German bath duck, personalised nutella, swim hat, swim goggles, fruit, sweets,
Age 9:
Dennis the menace jokes. Twisty laces. Scarf and Gloves. Pen with a bear climbing on it. 2 Mickey Mouse pens. 2 lego books, set of 3 Harry Potter books, 2 Minecraft books,
Age 8:
Snowglobe, balloon helicopter, few stocking fillers (slime, "pocket money cars" etc), angry birds rubber & topper, cube bot, Harry Potter book, Grinch book, snake pen, harry potter glasses,
Age 7:
Hello Kitty pen, moshi monsters stickers, hairclips, blue top, moshi monsters zipper pull, bouncy ball with flower, Christmas book, "Where's the Stig" book, transfer pictures, Harry Potter book, Jacqueline Wilson book
Age 6:
Buzz lightyear facecloth (needs to swell in water), mini parachute, 1 Mr Maker set, book, dollie nappies, dollie shoes & socks set, chocolate money, glow in the dark stars, meccano helicopter, small jigsaw,
Ove the years, the odd thing hasn't been played with, but mostly they have been successful. Funny, I remember slinkies being in there, just not on my list (from my annual spreadsheet of purchases).
We have a few useful things most years, and a few fun things. Keyrings tend to get broken over time, but that's because she uses them a lot and they are thrown at the bottom of the schoolbag etc.
I look in places like science shops and the shops in art galleries for interesting, quirky bits a lot of the time. But have found good things in Tiger as well (before DD discovered it - now Santa can't really go there anymore).
Useful practical things like carabiners, torches, buff head scarves, peaked hats etc have all been very welcomed. (She's a Scout and does a lot of sailing too, so there are always small things that can fill space related to those). Plecs for guitar, individual sheet music pieces for fun not exam/lesson ones....think of what your DC enjoys or the clubs they go to, and what might be useful there. I think DH has thrown in a new gumshield on more than 1 occasion! And shin guards. And fun long socks to use for training (rather than always the sober club ones for matches).
As she gets older, more grown up versions of things she still likes are good - so instead of small playdoh cutters, a couple of proper cookie cutters in interesting shapes are good, or a decent set of measuring spoons, or proper but small sized tools (screwdriver, hammer, measuring tape etc) for DIY with DH, etc. Or more adult versions of things like paintbrushes rather than the children's basic types. So things that develop with the child.
As she still loves her baths, and relaxing with some bath toys, a new Duck or other toy for water play can be good too.
And craft or other bits that will be used up - paint, glue, nice markers or colouring pencils, rubbers, drawing pencils, nice pens for school, beads, glitter, paper/card, feathers, baking ingredients etc - are good as well.