Have each person put a rock in their shoe, don’t tell them why. (I bought a small bag of fish tank rocks for this, all you need is one for each person, so if you have a fish tank you could take a handful from there). Then take a walk around the building or inside the church. Use this experience to teach that Sin affects us like the rock in our shoe. When we feel it it hurts, we can temporarily move it so that we don’t feel it, but it is still there and will creep back eventually. We have to remove it from our lives just like we had to take the rock out of our shoe.
I did this with my high school Sunday school class. It was a great introduction to our discussion of sin, repentance and forgiveness. We walked outside and around the building so we wouldn’t disturb the other classes. Another variation for youth groups would be to give them the rock in the morning at Sunday school and have them keep it in their shoe all day until youth group that evening. Then have the discussion at youth group.