Guests throw darts at balloons with paint inside. You keep the art. :) This is such a fun Idea. You choose the colors and the size to coordinate with your style - might be more enduring than having a fingerprint tree in your house forever.
Guests often forget to write their name on the response card. With this invisible ink pen, just number your guest list and write that corresponding number on your RSVP card. Then no one feels bad about the "number" on their RSVP card!