They started including codes with the certificates in October when the game first opened. Most codes are not reusable, but the ones I posted above are. They were promotions from
National Geographic Kids and school milk cartons.
Another code:
j955-k8s5-3wx9 will get you a nifty dance move

(this is reusable as well)
To enter these codes, go to the BuildaBearWorkshop room in the game (in the town square) and click on the cashier.