Themed Activities
You may want to pick a theme for your lock-in that relates to its purpose. If your theme is activism-related, you may want to include activities like making signs or writing chants. If your lock-in is just for fun, then you can think up a fun theme and play games that relate to your theme.
Spa Treatment
Depending on the group, adolescent girls may enjoy spa activities involving manicures and makeup. If you get a wide array of makeup and nail polish, girls can have fun giving each other spa treatment. If you wish to take this to a professional level, you may also be able to hire a makeup artist to teach younger girls some makeup techniques.
Scavenger Hunt
A scavenger hunt is a great lock-in activity if the lock-in is in a larger building. Plan a scavenger hunt before the lock-in and hide clues around the building, leading to a treasure chest. It may be best to provide prizes for all participants, in order to prevent competition from escalating, and to maintain the fun atmosphere of the evening.
Small Group Games
As the party winds down, you may want to split the girls into smaller groups in order to create a calmer atmosphere. Board games and decks of cards may help you to do this, as they function better in small groups, but continue the sustained and structured activities.
Late-Night Movie
The later it gets, the more necessary it is to provide quiet activities. Some girls may want to sleep, while others will need further entertainment. A late-night movie will allow for adolescents to get cozy if they want to sleep, and will give the rest of the participants something quiet to do for a few hours.