Buy one black plastic cauldron per attendee from your local party store.
Or even better, buy your cauldrons around Halloween if you know you're planning a Potter Party. There will be a wider variety of sizes and prices available and you most likely will not have to go to a specialty store.
The cauldrons can be used by attendees to store their things from the party when it's time for them to leave. They are also great for whipping up the most original potions only kids can think of.
© Jennifer Ofenstein (ofenjen)'s suggestion was taken with permission from Sewhooked.org