Definitely your holiday homework. You probably got a set of assigned questions, but if you find you have time you should do all the questions from the chapter(s) you were assigned, to help cement the concepts. After that, making your bound reference is a big priority; and that includes all the annoying little details like deciding how to organise it (colours, tabs, content/examples, etc.), when to work on it during the year (daily, study periods, weekly, monthly, before the SAC, before the exam...?) and so on. If you felt so inclined, you could make your bound reference right now, but it wouldn't be that useful because it'd be like 99% textbook. Other than that, you could write programs for your calculator, or write practice materials ("topic tests", practice SACs or even exams) for later in the year to review, or to sell off (or share for free

). You could even just work ahead to get an idea of what's coming up during the year if you want.
You're only really limited by your imagination, good luck!
