As the design is printing, I took a 6mm ball berring (slingshot ammo) and plopped it into one of the cavities. Now, the only way to get the ball out of the cube without breaking the cube, is to roll the ball through the internal maze and listen for clues. I have attached an image of the internal structure which could either be printed on paper as a clue, or if you're feeling generous, you could try to 3d print the internal structure as a clue (I have not tried printing the standalone internal structure, if/when I do, I will upload the .stl file and pictures of that as well).
This design is the first in a series of designs I'm working on to create puzzle games that require the gamer to rely on their sense of hearing, touch, and spacial visualization. As the first in the series, this design tests some of the boundries that I knew would be part of other designs and as such this puzzle is EXTREMELY difficult. This is a perfect gift to give to that special someone in your life who thinks they can solve any problem.
A note about the ball berring. You could 3d print a 6mm sphere, or use a 6mm marble if you do not have this part. Just be sure that the gap you're dropping it into mid-print has enough clearance that it will not interfere with the new layers. Another way to go about this is to wait until the print is completely finished, then put the ball in, but this will likely decrease the effectiveness/difficulty of the puzzle.