I think the best thing would probably be to pick a few topics that look really interesting, and just start learning about them for fun, I guess. Really delve into them in ways that won't be possible when there's the HSC coming up and you have to study what's going to be on the exam. That way you don't have the stress of full-on studying (writing notes, obsessing over the syllabus, cramming down information) but you still carry on learning informally, and by the time that topic rolls around you'd have developed an interest and some knowledge in it, which can only be a good thing, right?