I've been Trick or Treating in Second Life.
The first place I've been to was the Dreams sim. Once you get there set your enviroment to midnight.
Trust me you'll love it as you can see.
The second place I trick or treated was the Bare Rose sims.
Look for this display around the main club floor. It's setup as a sci-fi-ish hunt.
It's sooo cool. You get a great outfit.