Some volunteer work with the BC Children's Hospital (in terms of the research institute) or internships with a medical center can be beneficial as it can provide an insight in the many fields available that you may be of interest to you.
But consider supplemental activities as well. Imagine reading your application from someone else's perspective. What is your resume saying about your brand? What is the story you'd like people to understand about your life, interests, and competencies? Participating in activities that are outside your career concentration shows that you have passion for diverse topics, and almost always these opportunities build transferable skills.
In school, you could do something like Science Olympiad, which has many different events you can try depending on your interests, robotics, biology club, or others.
For college admissions, you may also want to get some leadership positions because they also look at character when making admissions decisions.
Outside of school, you could shadow at a hospital for some medical experience, though I think that is more geared towards people who want to work with patients in the future. I'd recommend doing research at a university related to bio-medical engineering if possible. You can then use your projects/research to enter for competitions like BioGENEius or others, which will give you more presenting experience and look good on your applications.
Super excited for you as you think about taking up a career in a STEM-related field. In very general terms, I think your efforts towards focusing on doing extracurricular work, with a goal of bio-med engineering in mind, is to consider volunteering. Some suggestions may be to volunteer at a local community college that has bio/bio med labs. Maybe there are some online opportunities for you to volunteer as a proof-reader for bio-med/engineering materials (proficiency appropriate materials of course).
In some instances, local community schools offer summer programs to students who may be interested in biology. Starting out, you may find that searching and getting opportunities in biology work is a bit easier than specifically bio-med engineering. They key would be to take small but consistently progressive steps to get the exposure and knowledge needed to give you good grounding in this field that you have an interest in.
Hope you find this answer helpful. Best of luck to you!