Once an hour during Hallow's End, each of your characters can talk to an Innkeeper and do a "trick or treat" option that appears along with the normal Innkeeper options of buying goods or setting this Inn as your home.
When the Trick or Treat function is selected, something random happens to your player - either you're awarded a Treat Bag that has something special inside, or you're buffed with a "costume" buff and your appearance is changed for an hour of play time.
So far, the cosumes I've seen have included:
Skeleton Costume - like that from Noggenfogger Elixir, lasts for 60 minutes, is a shapeshifter form that one cannot mount up while in. Has a /dance related to it and no battle restrictions, so you can play and cast normally.
Ghost Costume - turns you humanoid and faded, like the ghosts in Eastern Plaguelands for Pamela's Doll quests. Has a /dance related to it and is a form that you can fight in without problem - I haven't tested the mount capabilities yet.
Trick! - turns you into a small creature of some sort (red imp, grey borer, black cat) for 30 seconds, but you are not able to attack or /dance or mount or catch the wyvern or do anything other than run around and try to gather attention before you morph back to yourself.
Bat Costume - cute little bat that lasts for 60 minutes - instead of running around, you soar and fly around - at ground level, as if you were walking or unable to hover more than 2 feet off the ground. Unfortunately there's no /dance, and you can't mount nor ride the wyvern or cast any spells in this form.
Wisp Costume - remember when it was rumored that the Wisp would be a new class type for Burning Crusade? Remember how we scratched our heads and wondered what our equipment would look like on our Wisp toon? Well, turns out that was just a big joke, but the Wisp lives on in this 60 minute costume. It doesn't allow for any special /dance, nor can you mount up or take the wyvern or cast any spells while in this form.
Pirate Costume - there seem to be a few different "looks" to the 60 minute Pirate costume - different hair colors, clothing colors and the likes, but they all are human and have the same dance and they all mount up just fine (and ride the wyvern, too) and you can cast and do battle with them no problem.
Has anyone seen any (from the Innkeeper) I've missed?