What are some fun, team building activities for 11-15 year olds?

Ultimate Frisbee! It's fun and requires teamwork and communication skills. All you need are 7-15 people split into 2 teams, a Frisbee, and a simple soccer field to play on. Each team is on either side of the field. One team will throw the Frisbee as hard as they can to the other team, and the other team will try to get the Frisbee to the other side of the field. The person holding the Frisbee can only take two steps, and must throw it to another teammate. If it lands on the ground, the opposing team will take possession of the Frisbee. Other players can interfere with the pass as long as they don't slap it out of the player's hand. It is fun, gets the youth good exercise, and teaches good communication and teamwork skills! It's a game that my youth group played all the time when our lesson plans got shot down. :)