1. Make candy apples, but use onions. As long as the coating is thick enough, you can't tell the difference until you bite into it.
2. Put out a bowl of M&M's mixed with Skittles and Reese's Pieces. That's a classic.
3. Make a small hole in a Reese's Peanut Butter Cup . . . take out some of the filling . . . and replace it with mustard or Sriracha.
4. Unwrap a few yellow Starburst, and replace the candy with cheese. Just make sure it doesn't get mixed into their candy and sit there for a week before they open it.
5. Take a box of something like Nerds or Sour Patch Kids . . . swap out the candy for a few fake bugs . . . and glue it shut.