I don't know if they have anything special going on, but there are lots of people in costume. They supposedly don't allow Disney characters (can't have the public seeing them getting snockered), weapons or masks that cover the face. I don't remember seeing any Disney characters, but I do remember a Star Wars stormtrooper with his face covered.
Last year I was in the Adventurers Club and didn't leave it, so I don't know if anything happens outside or in the other clubs. Used to be they had a costume contest. I don't remember any announcements for that or anything last year.
I'm going to be in the same place this year.