Even if you have a résumé, you must still complete the Activities page of the Common App.
List your activities in order of importance (from most to least), and list only your most important activities. Too many activities can weaken your application, making you look like a dilettante. (Good SAT word!) Even with room for up to 10 activities, don’t list more than nine. And remember, activities are not limited to those in which you participate in school; include outside activities such as work, internships, community service, summer programs, and special hobbies.