Depends if your an ethical person or not lol. Well it's the HSC and i think everyone wants to get the most marks they can haha even try scabbing if you have to :P
Calculation for hsc is like the same thing you did for prelim with maybe like an extension and require more scientific thinking. Other than that i think it's fun :)
Umm you can do the xylem/phloem experiment. Buy like a celery stick from the groceries and put in some like dye or something to investigate the movement of substances and to observe the transverse/longitude cut of it.
Try take a look at some of the past papers including this year's if you have. Teachers tend to just get the question or change a little here and there from it.