For the past two summers, I’ve attended a week-long academic summer camp where campers stay on a college campus and go to classes (not college classes, just educational classes arranged for the camp by counselors). Could I count this as an extracurricular on the Common App?
Yes, although hopefully a week-long camp is n to among the most meaningful of your EC’s.
Yes, ANY activity that you do OUTSIDE of school academics is an EC.
And you should have some ECs that show how you spent your summers.
I actually disagree. You would list it as an activity you did over a summer (if the application has not changed then thee is a section for that). It would only be an EC if it is something you did consistently. As an example, if you do photography regularly (as a hobby or do it for your school paper) and took a summer course then you could list photography as an EC and also list your summer course. On the other hand, if you went to a one week summer camp for dance and never did it again then it is not really an EC.