I'm not an international student so I can't really give recommendations. I would assume that it's more difficult applying as an international student than a US student and, therefore, you have less leeway on your application (i.e. your app needs to be stronger all around). But again, that's just my assumption. Sorry, I can't really help much here.
I have no ties to the adcom or anything, so I can't tell you what Nova is actually looking for in an applicant regarding EC's. But as a general guideline, quality > quantity, and of course doing things that are unique also help to set you apart from every other applicant. Most people in my class volunteered at a hospital/clinic, did some kind of research, and shadowed a physician. Less people had a different career before applying to medical school, got a second degree, served in the military, are raising children, did international volunteer or medical work, started a business, etc. It's important to have medically related ECs, but I think it's also important that you do things you enjoy or have a genuine interest in- those things will make you more unique. I was asked more about my hobbies and non-medically related activities than my hospital volunteering/research during interviews.