Curated OER
Kids Health: Extracurricular Excitement
This is a very general overview of the benefits to be obtained through joining extracurricular activities.
The College Board
College Board: Extracurricular Acitivities; Life Outside the Classroom
Here are some thoughts on how colleges look for extracurricular activities or a "passion for an activity outside of the academic setting."
Scholastic
Scholastic Parents: Best Bets for After School
Some solid advice on extracurricular activities: why they matter, the benefits, the activities to choose from.