Dos Equis and Margaritas say Cinco de Mayo to me.
Sorry, I'm not much help, huh? I'm an avid cook and an incorrigible baker, so usually i've got a dozen ideas. But Mexican appetizers (guac, tacos, etc.) and alcoholic drinks are all I've ever had for Cinco de Mayo. Oh, and Birthday Cake - one of the kids in my Sunday school class had a May 5 birthday.
I'm going to think about it, though and see if I can come up with any ideas. It sounds like fun.