Absolutely. In fact, as far as I'm aware, some schools will even allow you to substitute completed A-Levels for SAT Subject Tests -- do your research on this point!
As to your second question: anything you feel either a) shows commitment to your academic interests (Economics, I assume Maths, etc.), or b) proves you to be a multi-faceted, well-rounded, 'has a life outside of the classroom' individual is great. As you probably know, US institutions put a fair bit of weight in the application process on the strength of a student's
extra-curriculars, and as such you really do need to think about being able to provide them with both a quality and quantity of activities (drawn from your last three/four years) in this area.
Jobs are great (not least for helping to cover the application costs, visa, etc.!), as are
volunteering and
leadership roles of any kind. Are you the
first/only to have done something in your school? Nationally? (Don't worry if you're not!) Or... do you regularly attend lectures? Care for family members? Cycle? Whatever it is, it's just another way for admissions officers to
meet you as best they can... and BIG emphasis on 'you'!
In short: whatever you like, so long as you're doing
something!
(And just as a matter of pedantry -- though I suppose also for the sake of clearing things up! -- you would
not be an Economics applicant. No-one is. On the whole*, students apply not for any particular course, but instead simply for the universities themselves. 'Declaring one's major' comes later -- there's a lot of flexibility!)
Hope this helps!
___
P.S. Have you considered looking into applying to the Sutton Trust? I'm honestly not sure where you stand from an eligibility point of view being in year 13, but it's really worth looking into! They're amazingly helpful.
*There may well be exceptions here, but I can't say I know of very many... Caitlin?