What a great idea! I found a list here on this website:
http://www.hiddenmickeys.org/WDW/Official/official.html
there's quite a few listed on here.
And also there is a book called Hidden Mickeys: A Field Guide to Walt Disney World's Best Kept Secrets by Steven Barrett.
amazon has it for $8.36 (plus shipping) but i've seen it at my local B&N also for a little more with no shipping. I'm pretty sure they sell it at the parlks and downrtoewn disney also. you could get the book and give everyone a different color highlighter, and as they find them, that person uses his/her highlighter to mark it in the book. Also ask the CM on the rides and in the shops if any mickeys are around....they always were great with us--not necessarily telling us out-right, but giving us look here type hints!
You could also ask one of the DISigners for a hidden mickey t shirt DISign that says "I'm huntin' mickeys" or something, and i bet people would see it and help and point them out!!
Here's a few to get yall started: