To go with the cereal killer costume idea, be the victim.
Wear any clothes you don't mind ruining, something you would normally wear.
Glue together cereal like Fruit Loops or Corn Pops in bunches and then glue them to your clothes. Add red paint or fake blood and spoons.
I also added body parts sticking out of me with bloody spoons. Paint face pale white with blood and cereal all over.
Get creative. Go as a couple. Killer and victim.
Thanks to Wammara in Ontario, Canada for this fun idea!