On my old wrestling site I held a scavenger hunt. 
I made up three images which spelled out a 
phrase. I was able to get official wrestler site 
to help me out and they hid the images on their 
website as well. Whoever could find all three and 
send me their URL's won the prize, which was an 
ECW T-Shirt and Video Game I believe. If you 
could get some other sites to help you out and 
hide some images under links on their sites then 
you could have a really fun contest. I think it 
took about 36 hours for a person to find them 
all. Have fun.