Pretty damn good idea, very simplistic - almost too simplistic interface, however, you need to provide more detailed instructions as to what to do to find the items in question. Post links as answers, search a picture for hidden items, etc. The easiest way would probably be to place a "mock" Q&A.
You could also use some custom visuals, question marks, 3D maze images, anything that visually transports what your site is about.
Bring out a list of winners, not listing their emails, but listing their first names.
Find out whether you can extend your scavenger hunts to be hunts with time trials. Bring hunts where you're looking for specific items related to a topic and see whether you can find sponsors,....there's a boatload of opportunity there. Great Idea!
A few modifications, more visual action and you have a solid winner! :tu:!