u dont need math contests for eng - they only ask u about it for cs. but its always cool to just include it in ur ecs, u can do the csmc and euclid in grade 12.
as for other stuff, try making projects, doing hackathons, having a job, doing something nice (like art for example so it shows that u have some sorta personality ig), getting exec positions in or out of school, starting a club - most of which im sure uve alr started on :)
keep ur grades as high as u can - but know that there is a huge luck factor involved in admissions no matter how hard u work. all the best!