If you are planning a birthday party on the 31st. Perhaps consider having a haunted birthday. Perhaps a sleepover where everyone comes dressed up and they all arrive a haunted mansion. Perhaps, a basement is where the haunted mansion is...
Play haunted type music, play games, have a ghost show up and have weird or haunted things happen to the guests all evening. Like a mystery that the girls have to solve.
I liked your idea. We actually decided to have a mystery cruise. The guests were invited to participate in a fashion show on an island but to get to the island they take this cruise. During the cruise to the island we will make up their hair, faces and they get into their outfits. After or during the fashion show they find out that the cruise ship was taken and they need to find out how to get off the island. They will go to the "natives" and get clues to figure out how to get their ship back and get off of the island.