Furthers a funny one because there isnt too much to learn, and it isnt too difficult, but to get the top marks you need to do a fair bit of practice questions so that you remember the tricks in different questions
id say that if you wanna be productive on the holidays, youd be better off doing ANY other subject other than further. as you will be able to pick up further in class without too many worries (if you need help come to atarnotes!)
if you are dead set on doing further then i would spend an hour or two getting a feel for all the modules you are doing, and if you are gonna do some proper work, get cracking on the first module you are going to do at school. if you finish that chapter of your textbook or whatever, i wouldnt go ahead to the other topics (except maybe core if you want). but i would just start smashing out practice questions and practice sacs for that first topic until that first sac comes up. alot of the marks that people lose in their further sacs are in the very first sac when they arent ready for those tricky questions that separate the 100% from the 98% and 96%.
id say thats the best course of action, but if i could go back to my summer holidays before year 12 i wouldntve wasted my time doing any further
good luck for the year