There's really not much you can do. Just read through the syllabuses for each subject so you have an idea of what you're going to be learning, read any texts you've been given (e.g English texts), make sure you set up a filing/organisation system for each subject and that you have the appropriate equipment (i.e folders, plastic sleeves, etc), and that you set your long term goals and create a study schedule that you stick to from the very beginning of year 12 (because consistency during the year is super important).
Though mainly you should just take these holidays to relax, because you really don't get a chance to during your HSC year. You should be more concerned about how you're going to use the 6 week break over Christmas/new year (because it's much more crucial that you use those holidays well.). Best of luck for the year ahead, as long as you consistently work hard, you'll do well.