1. Create some ‘PLAnetary art’

Every good prank needs to leave the victim feeling completely bewildered. And nothing instills this feeling quite like using orbital lasers or ship landing mechanisms to carve random patterns onto the primitive’s homeworld for no reason at all. Crop fields are particularly malleable and make an excellent blank canvas.

Bonus points if you can create intricate designs that look like high-end art, but are actually highly offensive obscenities written in a xeno language. If they survive, primitives will laugh out loud several hundred years later when they uncover the true origins of these cheeky doodles.