Being the avid fan of Halloween that I am, I wrote a case for Sherlock a while back that takes place at a costume party. It's completely independent of the books, though it probably helps if you've read book one I guess. And yes, the blatant references to a particular board game are entirely intentional.
And after you're done at the costume party, why not go trick or treating with the Irregulars?
And after you're done at the costume party, why not go trick or treating with the Irregulars?