Create a puzzle that will require the cooperation of all the recipients to complete. The contents of each drive should be tailored to the individual. Computer-savvy people could have an encrypted document or image on their drive; computer dunces could have a simple text file that says "Call Joe at 870-555-1234 and tell him to give the password on his drive to Mark at 901-555-4567." Put hints on some drives, and images, and text files, and passwords, and instructions that, if all are followed, will result in the final unveiling of something cool.
The "something cool"? I don't know. If you have some money laying around, it could result in uncovering a bunch of $10 iTunes gift certificate codes on some web site somewhere. (But it'd have to be done in such a way that each person involved can claim exactly one certificate.)
Ideally, build some redundancy into the puzzle so that even if 10 or 20 people don't participate, the remainder can still get something cool in the end.
If you choose to do this (and I must say I think my idea is pretty awesome), keep me posted on what you do. My contact info is on my /. profile.