Ideas

What i usually do is put  candy they like; like pez and pez dispensers and little games, a deck of cards with characters on them, uno cards, chapstick (my son loves it), plastic animals (tigers and lions and stuff that they sell at target and ac moore), coloring books rolled up.  Little things like that. You could probably find stuff at the dollar store too.  Nothing fancy just a bunch of little things. We wrap them in wrapping paper too just more stuff to open.

i usually find most of my boys' stocking stuffers at the 99cent store. No kidding they have some awesome stuff. Like... Spiderman yo-yo and cards. Winnie the Pooh crayons, and stickers.

I also put a new toothbrush for each of the kids, candy, a couple of racecars(like hotwheels), my oldest always gets new pencils. and i try to find something small that may mean something to each of them, like last year i got my oldest a harmonica and my youngest got a couple of Elmo board books.