I think it's really important that we take time to relax during these holidays, especially after those 3 daunting terms. We need to regain our energy after all! If you don't relax, there's a greater chance you will burnout later. It is the worst state to be in especially during the HSC itself. So take these holidays to: watch stuff, go out with friends, play games or anything else!
Of course, it's a good idea to get a head start on year 12 but as the person said above, don't go crazy as these are our last holidays before year 12! So for your subjects: English advanced : Honestly, I am unsure how to prepare for this either. I have no clue what texts I'm studying next year but if you know, probably try to start to familiarise with them like the main ideas, plot, characters etc
Mathematics Extension 1 : Since 3U topics build upon 2U, get a head start on the 2U topics. It's also crucial that you focus on improving your weak topics from year 11, especially for the extension topics as the HSC tests the entire course.
Business studies : This is probably the one subject where I would rather wait until school starts since I only get my textbook at the start of the term. You could consider looking up the syllabus and familiarise with it.
I don't do chemistry so I am unsure about that haha. Again, you could look at the syllabus. If you have access to textbooks or other resources of the new syllabus, try to get a head start.
Otherwise, make sure to relax a lot during these holidays! It's really good you are willing to study ahead but remember that these are the last holidays before the actual HSC.