Thanks to all for the comments. I thought there would be more posts to the question, but quality is better than quantity, anyway. Posting a cache at all airports is a great idea. Next time we're at the airport, I'll check it out.
I was also thinking about an event cache -- similar to a road rally. Area pilots would fly to one specific airport. Get their clues the the caches which would all be in/around the airport. I would rotate the order that participants' clues were in so everyone wasn't going to the same place, then hunt. Eating would, of course, be involved after the hunt. To do this, though, we would need enough pilots willing to participate in and around SE Michigan
I'm trying to get DH to post a question on one of his many pilot boards, but I think he's embarrassed (he's a purist)!
Thanks again to all -- we'll see what transpires.