Here's a great prayer idea if you don't have access to a full-sized labyrinth.
I have always found it meaningful to pray with a labyrinth. The insights you receive through the physical act of walking and praying as you traverse the path of a particular labyrinth can be quite surprising. I have found that youth are naturally drawn to labyrinths, perhaps because of their visually creative designs and because they offer an opportunity for active prayer. But you may not always have access to a large-scale labyrinth. I think this idea would be a great substitute, providing youth with printed copies of blank labyrinth designs and inviting them to "walk" the labyrinth by writing in their prayers as they move through the design. Challenge them to use different colors to represent different emotions, thoughts, insights. This activity would be best done in silence, perhaps with quiet music playing in the background.