Someone told me that I should read the whole chem textbook over summer, and I did...complete waste of time (for me). In the holidays before Year 12, I think it's best to concentrate on having holidays and for subjects with midyears like chemistry, don't even look at unit 4.
It would probably be a good idea to get your head around the first couple of topics though - if you can get to the point where you can do a chapter review relatively easily, that will help you a lot!
In terms of what to do during the year, you'll probably find that you work it out for yourself. Personally, I read ahead and tried to do the odd question from my textbook before we learnt a topic, then I wrote summaries in class while the teacher was talking instead of/ as well as copying from the board. That saved me time and meant that when it came to a SAC, I never had to write summaries or learn concepts - just had to do some extra practice questions. One thing I recommend: BUY NEAP SMARTSTUDY. It has about 2-3 pages of questions for every AOS, and the Q style is very similar to exams and gives you a good indication of where you are at with your understanding. Normally I find these kind of books a complete waste of time but I thought it was really useful.
I got a SS of 40 and I know you're aiming higher, but these things might be helpful anyway...