I would recommend using these holidays to go over old exams, to look at where you went wrong so you can improve in these areas and maximise your marks! This is especially important in maths where content from prelim carries on through to HSC. It is important to make sure you have a strong foundation before starting the HSC course so make sure you understand the solutions to questions you got wrong in exams. In other subjects this can give you an idea of what will be most beneficial to work on. Say in chem/phys if you always lose marks in long response questions you could spend some time these holidays creating scaffolds for answering specific types of questions. Or if its calculations you lose marks in, is it because of silly mistakes? Or do you have trouble transferring the words of the question into an equation? Asking yourself these types of questions will help you focus on the areas where you are losing marks and sorting these issues out now will help to improve your marks for next year!
In terms of content, depending on how much time you are realistically prepared to put in these holidays, getting a head start is always going to help. As Sp3tre said reading your prescribed texts for English now is definitely a good strategy. Possibly even starting to think about additional material (especially for discovery) would also be a good idea. For other subjects reading ahead (in the textbook or using notes you find online) will definitely help you once you go back to school. I never really did this during my HSC year (I waited until content was taught in class before making notes/doing questions) but I know many people who did this and found it very helpful.
As for preparing for multiple exams at once, this is something I always struggled with. Think back to how you prepared for exams this year. Did you spend too much time on some subjects and not enough on others? Did you not devote enough time to the subjects you found most difficult? Did you feel rushed and unprepared leading into the exam period? Learning from things that haven't worked for you in the past, being prepared well in advance of exam periods (working consistently throughout the year, including regularly attempting past paper questions, making notes, writing essays, etc.) and working out a study timetable in the lead up to exams is the best advice I can give for this.
Hope this all helps! Remember the HSC is a marathon, not a sprint. While working these holidays will put you in a good position for the start of year 12 it is also important not to go too crazy and to take some time to enjoy yourself so you don't burn out. Best of luck for your HSC year!
