Spin Box Puzzle
Update: I recommend using this design instead, as it works better with the box turning upside down, etc.: https://www.thingiverse.com/thing:763835
This is a box that only opens after it has been spun counter-clockwise! Any other motion does not allow the box to open.
This requires eight 6mm balls or bearings. I have used airsoft BBs purchased at Walmart (see the last gallery picture). I suppose you could try to print your own 6mm balls too, though I haven't tested that idea.
Inspired by this box:
http://www.instructables.com/id/3D-Printed-Centrifugal-Puzzle-Box-solved-with-/
The box above will open when spun in either direction. My box only opens when spun counter-clockwise (which is not the way most right-handed folks spin things).
This has now been tested and works like a charm!
Video of how it opens:
http://youtu.be/nhxeBcmDFvA