I definitely agree with the person who said go over basic algebra... I wish I would've done that, it would've saved me a lot of time at the beginning of the year trying to understand things that I'd forgotten from GCSE. Only other advice I'd say is buy yourself a lever arch file for each subject and get file dividers so that you can be really organised, it helps for when you need to revise for exams and you need to find a specific page of notes. Other than that, have a rest over the holidays because next year will be a bit of a shock to the system and it'll go really fast.