I snagged this from patterns_rhyme  who got it from foxrafer and stormatdusk 

On October 31 create a Halloween post and invite people to "trick or treat" by leaving a comment. You can give any kind of treat you like: a picture of their favorite actor, a manip of their OTP, an icon, a ficlet, a few words about why you're happy they're on your flist ... basically anything goes.

Be as creative about your post as you'd like ("decorations" are encouraged!)!