In addition to having a guest, a really fun way to do a similar thing is with a neighbor about the same age. Don't tell the party guests they are solving or capturing anything. This idea works best with sleepovers. Have your neighbor set odd objects on the porch of the birthday girl/boy's house. Or even have them write some mysterious note onthe sidewalk w/ chalk. Make all the spooks related some how. But make sure that the kid's don't know about it. Just for effect have the assailant wear a black hat and make noises. This may go on for a while, all the time the guests not knowing about the person. Gather up some flashlights and go on a "witch hunt". In the morning or later on that night have an exciting conclusion to the mystery. Have the person sneak up and surprise guests or think of a dramatic ending. It's fun either way. Teens will especially love this idea. Tape it for laughs afterwards!!!