HP People
What a jolly idea! Let me see... perhaps we should follow in the footsteps of those inestimable pranksters, the Weasley twins. The most creative and amusing use of a simple spell or potion!
Rules! Yes, we must have some rules. Firstly, the spells or potions must be simple. The sort of spells or potions you learn in your first two or three years at school. That will keep things nice and level on the playing field. Secondly, you must have the consent of anyone you use the spell or potion on. If they say no, you will respect that. Thirdly, nothing that will be harmful to Muggles. Fourthly, the effect of the spell or potion must wear off within an hour.
I think that covers the obvious thing, yes? Play nicely, in other words, and be creative and funny!