I absolutely love Chick-fil-A. I've never done Cow Appreciation Day, but know someone who has for many years. She and her kids get dressed up as "cows" and go get their free meals. They think it's fun and they take pictures and the whole bit. They usually just do something simple like put black dots on white shirts. I don't think you have to go the full cow route!
On their site is info about a facebook contest, a starter kit, and FAQ.
So start practicing your moos and get ready for some free chicken! Enjoy!