Two apps were downloaded to each tablet: Ghost Observer and Ghost Radar. Ghost Observer allows you to view everything in a sort of grayscale and every so often a shape will appear and the voice will tell you a ghost has been detected. Ghost Radar looks similar to what you might imagine when you think of sonar images.
I also wanted to download Ghost Sensor because it looks so much like the EMF meter Dean and Sam use on Supernatural, but it was not available on Android.
The program took place after hours, so there was an extra layer of spookiness because everyone was gone and the library was empty except for the teens and two staff members. One of my coworkers had suggested she come in and make rattling noises and run around and spook the teens, which would have been tons of fun but she was not working that day.
While the teens waited and ate Halloween cupcakes and cookies, I hid some ghost plushes around the library. Then they split up the tablets and went off using Ghost Observer. If they found a ghost plush they could keep it. After traveling around the library, we returned downstairs and tried Ghost Radar. We had some time at the end, so I'd planned a candy corn toss game where they tried to get candy corn into small cauldrons for a chance to take home some other Halloween prizes.
This would also be a really fun activity with younger children since the apps are not too scary. My favorite moment of the night - when a teen looked at Ghost Radar and followed the red blob on the screen and paused before he said, "Wait...there's one on me!"