door prize

noun2

1. •
   A gift given to people who attend a meeting, party, event, etc., especially one which is given on a random basis to one or some of the attendees.

   • Dewey infantrymen passed out soft drinks and small favors to gawking visitors and gave every 200th visitor a door prize. — 1948
2. •
   (humorous) A notional prize awarded to a cyclist who collides with the open door of a parked car.