Create a victory condition where you can win if you do something but not another. (e.g.you win if you touch the edge but haven't defeated any mobs.)
Create a puzzle where the player can only do one thing but not the other. (like push, but not pull, or vice versa
Optional challenge:Create a character controller where the player can do one movement BUT not another
Incorporate a familiar secret, easter egg, cheat code, or similar hidden idea or object into your game
Optional challenge: Include a familiar sound effect or music track somewhere in your game
Optional challenge:Include a familiar intro, start, or introduction into your game.
Disqualified for being a teacher submission. :)