Just a neat little safe I made that only opens if you put a loonie in it (or any equal sized coin).

Originally designed as another gag gift for a friend, and after they had gotten through layers of wrapping paper and boxes, they finally opened it to only find 3 whole peanuts inside :]

Check the images for an idea of how to put it together. (it may be better to insert the long peg and hinge onto the vault before gluing the assembled door to the hinge though.)

Made in SolidWorks.