I think the biggest mistake that you could make is doing anything remotely school related in that time. I think reading school texts or other light forms of work isnt a productive use of free time. With the HSC, Uni, work etc you need to have time for yourself where you arent doing anything work related. Having that separation helps give you a balance and also allows you to be more productive when you do return to work. I know this sounds like a lazy answer, but the fact is a strong work/life balance is critical to success. Businesses spend millions per year on programs encouraging workers to create this balance because they know that it produces workers who are more motivated and productive.
Spend your free time doing whatever makes you happy - whether that be video games, bludging with mates etc. Just make sure it doesnt relate to the HSC in any way.