Hang on to your exercise and co-curricular activities for as long as you can, IMHO.
The more you do, the busier you are. The busier you are, the more efficient you become - because you become a master of productive procrastination. It's a trend that continues well into university. The best-performing students I know work hard but maintain a life outside study. And those are the people that everyone wants on their scholarship scheme, who get through the interview selections, and who everyone wants to hire. Well-balanced people. Of course, that's no excuse to ignore the pile of textbooks you ought to get through - you still need to do well in your HSC.
Just remember to look after yourself, get enough sleep, and work regularly and efficiently at your study plan. Find the balance that works for you.
p.s. re: What to do: if you're doing (1) well academically, (2) sport/outdoors, (3) music/cultural, (4) charity/fundraising and (5) student-representative/community stuff, this will hold you in pretty good stead for most applications.