Yes, fill the cauldron with a ball of foam. Get lime green candy (or fondant balls) . Make a really cool witches leg thing and be done. You can do the topper way ahead of time and be a calm and collected party host. No need to frazzle at the last minute. And it's a cute thing to save for a few days after.
Plop it in the middle of a dish surrounded with cupcakes and you have another family celebration moment. Cute and easy.