For my daughter's 11th birthday party we did a mystery party. We bought one of those "teen mystery party" games and invited 6 girls. Since it was a mystery, I made miniature file folders and typed the information in like a police report. We put a fingerprint on the outside to make it look "official".  Then we sent each girl a character identity and instructions on how she should dress. The girls did a great job with costumes and acting! We decorated the house like a high school, too. We put up posters advertising a dance and votes for class president. We made a menu and the girls went through the cafeteria line for dinner. We served hot dogs, hamburgers, chips, pizza, etc. My husband even dressed up as the "cafeteria lady" and "yelled" at the girls for talking too loudly! The party took about 4 hours to solve the mystery. The girls all said it was the best party they had ever been to! My daughter is still talking about it! I wouldn't recommend this for girls any younger than 11 though, it does take the ability to sit for awhile and figure out the clues.