Oh, and summer sales, and back to school time, are great times to look out for things to put away. (You only do plain pencils for school? - Santa might bring a nice charater one for writing with or a fancy rubber, that kinda thing).
A gift card can be useful too for older folks (tweens, teenagers, or even adults if there are adult stockings!), and I know last year I kept meaning to (but don't know if they still do them) get the paper vouchers for McD's that are worth a happy meal she can buy herself one day. Just never got round to it.
Do they have dolls or action figures? A new outfit for those is small.
Small toys like little bouncy balls, playdoh tubs, small dinky cars or toy soldiers, marbles, kaleidescope,
Accessories for a game they have, or expansion sets - extra lego blocks, a toolkit for action man, an extra food item/set for the kitchen/shop they already have?
A character toothbrush and toothpaste (encourage teeth brushing!!).
Pack of cards
Jewellery or makeup (suited to age, mostly a girls thing)
Pack of seeds for spring
Glowsticks - great fun on dark nights!
Blank or Thank you cards, and stamps (blatant blackmail!!) - very useful to make birthday cards for friends too later in the year.
Markers (especially if Mum normally only allows crayons or pencils!), twistables is another rarity in this house that Santa sometimes brings.
An apron to help in the kitchen, or small sized but practical tools (like child sized brush and spoon set!!).
Small sized cutlery or a nice cup/glass to drink from. You might have plenty, but sometimes an extra set is handy to have, or to help move from 1 stage to the next.
If there is something that they USE, get extra supplies of it. Like small marshmallows for hot chocolate, or plain coloured paper for art, or anopther pack of pipecleaners or coloured matchsticks or bottle of glue....replace that bottle of red paint that has just been finished (clever Santa to know about that being empty!).
Kiddie torches for emergencies.
Sorry, I tend to get carried away with ideas....