Undergraduate / Need advice on subject for extracurricular activity (a mentoring club) [5]

Since this is generally about about your extracurricular activities, you should write about the debate club since this is the one you have had the most experience with. The time you devoted to this club would also be a plus for the essay readers since it shows that you are a determined and persistent individual. The more you can utilize in your essay the better off you can be. Honestly, colleges do not really care much about passion. They care about leadership, and how you were a leader in whatever organization you were in. Accentuate on those aspects, and you will be on your way.

Good luck on you topic choosing.