One of our really creative hostesses came up with some very funny and very affordable prizes, to give out in addition to the Certificates. Here are her suggestions, and feel free to reply with your own suggestions:
Super Sleuth received a giant magnifying glass.
Best Costume received a pair of glasses with nose and mustache attached. Wealthiest Player received a grow-your-own credit card.
Best Performer received a gold-painted pig as a "Biggest Ham" award.