When I was in high school, youth group was the only time I went to church. I doubt this is common for Reformed youth activites, but even still, I would be sure to have as many serious meetings as all fun ones. Or more. I wish that I would have learned more in youth group, though at the time I am sure I felt satisfied with mostly fun.
If you have some kids interested in a small group, I would maybe do something like have them come to my house or the church or where ever to discuss this book:
The Fallacy Detective. (Unless there is no Bible study for them, then I'd just do a Bible study.)
I did not know anything about logic in high school and would have enjoyed learning how to refute bad reasoning, esp. if I was headed to a state college.