Are you travelling during the summer holidays?
If not, I'd strongly suggest trying to study ahead, buy/borrow textbook for Year 11 because it's never a bad idea to start early & get yourself more familiar with the content.
I can't tell much with Chem, but for 3U Maths, it's a good idea to pull out your Year10 textbook and make sure you're familiar with those topics. Try to solve exercise questions fast with hands cuffed behind your back (i.e. being able to mentally calculate even when you're anxious). If you couldn't, write down the proper workouts for a few questions & try again.
It's also a really good idea to start learning Year 11 Ext1 (along with 2U). Although it hasn't start count towards your internal score like Year 12 does, you get an idea of how to study in your own pace; If you take tutorials (or not), do you want to study topics with the same order the school will go through? Or do you want to do the ones that you're more interested or "more beneficial"? etc etc
Also note the change of syllabus & make sure you get the right edition of textbooks. Looks like in 4U they deleted conics, some mechanics, harder extension 1 (If you are caring so much with MXT1 now I think there's great chance you will be doing 4U)... Heck, with the current syllabus, I really think anyone getting 80%+ in Yr10 Maths exam should be able to accelerate 4U maths and get bandE4 if they study the right way...