I have 2 four-year-old boys who will get things like cotton ragg socks, small wooden train whistles, slide whistles, finger cymbals, maracas, some boxes of band-aids (they love them, lol), a candy cane, a few pieces of chocolate, small trucks & a toy school bus, wind-up robots, and bird whistles (the kind you fill with water & they warble) I had a $40 gift certificate at Land Of Nod and put some of it toward small instruments & toys as stocking stuffers.
I have an 8 year old girl who will get some candy, knee high socks, a bird whistle, small flashlight, some barrettes & hair clips, and some small things I've been saving (some since Cyber Monday sales LAST year, lol) like doll-sized backpack/school books/pencil case, doll-sized sleeping bag, and some ornaments/keychains I've picked up that are doll-sized things she can use as doll props (like a Starbucks hot cup & a small "milkshake," a mini Ugly doll, mini Etch-A-Sketch...)