It depends on the context.So everyone agrees
So from now on I will use: what did you do at summer camp?
Yes, that is also possible and normal. I am sorry that this thread is turning into a thread about the use of articles. Alvin n was originally interested in the preposition at in the question What did you do at the summer camp?And what if I added "a" in this Longman example and said "a day camp"?
He is too old for day camp and too young for a paying job.