This was one of my favorites. We would take the youth group to a large shopping mall or main street (depending if it's day or night) and all the youth leaders would dress up or disguise themselves somehow. As an example: a homeless man, a guy with a moustache, a girl with a pillow up her top to make her look pregnant. Some costumes can look bit ridiculous where the leader really stands out, others can be more subtle and blend in with the crowd. Also have a combination of youth leaders sitting in the one spot (like a cafe) and others wandering around (like a cleaner with a broom).
The basic idea is to split the youth group into teams and their goal is to 'hunt' down the leaders and get their signatures (or a stamp) on a piece of paper. The winning team is the one that finds all the leaders first.
A note of caution - ensure the young people are adequately supervised. Depending on the age of your group, you may need to ensure there is a leader (not in disguise) with each group for their safety. Also ensure the group is aware of the 'boundaries', and have an agreed time to meet back.
As a variation, you could combine this night with the Photo Night and get the kids to take photos of the leaders they find (in amongst other challenges).