Although medical schools in Ontario have no specific degree/course requirements (aside from UOttawa*), it is recommended that courses in these fields are taken to assist with the MCAT:
*UOttawa has specific requirements since they don't use the MCAT in their application process:
6 units in humanities /Social Sciences
6 units in Biology or Physiology
3 units in Organic Chemistry
3 units in Chemistry
3 units in Biochemistry
3 units in Statistics
All with a completion of a B/70% in the course
Diversity in extra-curricular activities is extremely important and you should consider having activities that fit various categories such as:
It can be hard to secure research positions as a high school student but it is worthwhile in grade 12 to start reaching out to professors and labs at universities for support
Health Related Activities
Ex. Art, Music
Volunteering with hospitals or health organizations like Heart and Stroke Foundation of Canada
Illustrating that are able to take initiative to guide others will set you apart from applicants. This can come in the form of leadership roles in clubs or creating an organization/club of your own.
It is important to show long-term dedication to a few extra-curricular activities so you can network and meet people that can vouch for your character and ability to be a physician in your medical school reference letters.
A key tip for extra-curricular activities is to have them align with the "CanMEDS" Framework. Ontario medical schools heavily rely on this template to determine the characteristics they look for in a physician. Finding opportunities to practice and demonstrate these qualities will assist heavily with the medical school application process.
Having extra-curricular activities that can clearly represent each of these categories will strengthen your application greatly.