The things that websites and shops tout as stocking fillers are usually nasty overpriced plastic toys that break straight away. What we need is a nice long list of things that will be cheap, useful and interesting. Obviously everyone already knows about patterned pencils and unusually shaped rubbers, but it would be good to have other stuff too.
I will contribute:
Plaque disclosing liquid/tablets. DS got a pack of these in a party bag last year and I thought WTF, but he and DD really loved using them.
Mini magnetic bookmarks, DS is a keen reader but gets very frustrated if his bookmark falls out and he looses his place so I bought him these and he loves them.