A trick maze cylinder!
The cylinder has an "entrance" and "exit" for the outer ring on the bottom and top respectively. The challenge is to feed the ring from bottom to top as fast as possible, or to remove the ring from the inner cylinder as fast as possible. Print a few copies and race with your friends!
See the instructions to see how to make the trick version. Note that the non-trick version isn't very hard. If you want a challenging non-trick version that you can actually finish, try one of these:
This is also customizable via the scad version. See the instructions in the scad file. The maze itself is pretty easy, because it's set up for the trick version.
To make the "normal" version, just print the testNubRing.stl and the InsideNormal.stl files, and then feed the ring through.
For the "trick" version, print the testNubRing.stl file first. Then print the InsideTrick.stl file, but STOP IT HALF WAY THROUGH AND FEED THE RING ONTO THE INSIDE CYLINDER IN PART OF THE MAZE THAT HAS NO EXIT! Then continue with the print. You'll end up with a version where the ring cannot be removed. Hand that one to your friend while you play with the normal one. See how long it takes for them to figure out that there's no solution to their puzzle.
It works best if you have 1 or 2 "normal" versions so the person with the trick one sees everyone else having no difficulty...