I recently learned about "non-transitive dice" and realized I had to own a set.
Non-transitive dice have a lot of interesting properties, the way the numbers are distributed on each die - one die will tend to beat the second, the second will always tend to beat the third, the third will always tend to beat the first. Efron dice have a similar "circular" pattern of victory - with four dice - and Grime dice have a pattern of victory with five dice.
I've included the OpenSCAD file, STL's for a set of non-transitive dice, Efron dice, and Grime dice - and a corresponding STL for each so you can print the pips directly into the dice using dualstrusion.
I would recommend reviewing James Grime's article on the topic of non-transitive dice. You could print each in a different color (which is a bit of a give away), but you could also just memorize the difference die.