We did the first 7 day Halloween Cruise in 2000 & it was terrific! They have stuff going on for the kids all night, a big costume deck party with prizes, a family party in Studio Sea & I believe trick or treating in the Atrium.
There is a thread going for current Halloween cruisers. Several people, I believe "Ginger" is a repeat Halloween cruiser. They can give you much more current info. The Wonder's activities many differ from the Magic. I would post over there for more info.
It is a great time. My DD loved it, but missed trick or treating with her neighborhood friends, so once was enough... We now cruise in Nov., or April.
Hope this helps!