I need ideas for pranks on Halloween!

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I'm going to a friends Halloween party this year, and me and my cousin, who is also going, have decided to think of a prank to pull on somebody, but the problem is, is that we can't think of one! So, please could you give me your ideas and suggestions i don't want something too mean and there will be boys and girls there :) And if it's relevant I'm 16 years old and so are most of the people that will be there, but a few are a bit older, up to about 22.