On the Stuff page you will find two small d.i.y. games: Rocks and Stones and Twelve.

Also on the Stuff page, you could make a popsicle stick abacus, if you suspect that someone wants a popsicle stick abacus for Christmas.
 

Print out two sheets, then cut out one of them. Curved nail scissors are helpful for the tedious cutting. Fold it up, using the second sheet as a guide, and either assemble it and put it on top of the presents, put it in a box, or leave it flat (sticking out of the stocking?) for the recipient to finish.

There are a couple of variations around. I sent out a cleaned-up version of this to the guys in the mailroom. They filed it under "G". Twits.

You may decide that origami or paper craft has its place on Christmas morning. How about gnome bowling?

If you're in the mood for a morbid Christmas morning you could slip Death at Samarra under the Christmas tree. It is a very simple bluffing/deduction game, and it is a free print and play game that I would print on cardstock. You could leave it unwrapped, or differently wrapped, making use of the picture and the tale. I find this game very seductive, but I haven't tried it, so I don't claim that it plays well.

Under the right conditions rattlesnake eggs can work well as a stocking stuffer, or sandwiched between a couple of presents. They're inexpensive too. But make sure to treat these with great care - they could be dangerous! Did you know that people have cut the heads off of rattlesnakes, and the heads still twist around and bite them with their poisonous fangs?

A chinese finger trap might be considered an awkward gift. On the plus side, you've got some entertainment in a small size. You could purchase it for as low as 25 cents, or you could even make one. In any case, here's a search.

I've given a four leaf clover pendant a couple of times as a present. The ones I got were rectangular, with no metal frame. The green box it came in made it look better.

I recently got myself (a replica of) an old-fashioned key, which could be worn as a pendant, that I adore. To each his own.

A pocket wildlife/plant field guide. "Iguanas of Idaho" or whatever.

A small magic trick.

Kinder eggs are great stocking stuffers. Little kids can enjoy the toys inside - I'll take that nice milk chocolate. They're not available stateside.

Wrapped gingerbread cookie. I suppose a candy cane, hanging over a stocking, is a good idea.
You can't go wrong with mandarin oranges.

Some families wrap stocking stuffers, and some don't. We didn't. Speaking of wrapping, nobody will blink in our family if coloured newspaper comics are used for wrapping.

Christmas pet peeve: When they put polar bears and penguins on the same Christmas item. I won't use any item that has that.

What is Santa's postal code?