Sudoku wrapped around a Print-in-place Cryptex (OpenSCAD source on GitHub)
A friend suggested trying a Sudoku after seeing my Riddle on a Cryptex. The easiest version would be an already solved Sudoku, but you must arrange the rings so that no numbers are repeated on any row. This one is a 6x6 Sudoku and there are also smaller 2x3 boxes which cannot contain duplicates either but they are not indicated.
Also a bit more robust than previous versions which makes for virtually no post-processing. The nice chunky rings turn straight off the printer.
It is also smaller to accommodate only using 6 digits but it's about the right size to contain a rolled up bank note.