Depends on the subject, but from my experience:
- English/Humanities: if there's a multiple choice section - start reading & answering the questions in your head (gives you more time for other sections). If not/struggle with essays - look at your essay questions, work out which one to answer and start brainstorming in your head so you at least have some of the idea
- Maths: Go to the section where you need to prove congruency/similarity and start working it out in your head (honestly no numbers involved). Or if you aren't good at proofing/can't work out the solution, either work out the MC section in your head (if you can), a section you find easy or go to the last pages of your exam where worded problems are found and break them down.
I know it sound like a lot to do in your head but you will have the advantage over others who sit there in their reading time doing nothing (plus it helped me so hey). But ALWAYS briefly read over your exam - identify the challenging sections and the easy sections and try to read every question so that you aren't wasting 5 minutes in the actual exam trying to understand what is being asked.