Hmm, well you should first decide which of the three you want to do, because those subjects are very different. Now, I am not sure about how the canada, or USA system works, but yes, some extra curricular is good. It would be better if it related to your subject. And also, any useful work experience is good.
If you do decide to do economics, read, read and read. That is one of the best things you can do. But in all honesty, all top universitites care more about your grades and marks than anyhting else.
Atleast you can display yours on your application, and if indeed they are good (again, i can not say if 93% is good in your subject, i averaged 95%, but my grades were all As, which you only need 80% for) then universities will not care as much about extra stuff. In the end, these intitutions are about studying, and learning. They do not care if you are brilliant at a sport or something, but they will care if you are indeed interested in studying above anything else. That is how it is supposed to work, and it should if you do not have a skewed system (like mine).
Good luck, and hope you get in.
G-man