Because I'm doing that. I can't help it now. Already bought my ticket. I know I can't work before my study period starts...but I can volunteer, right? And that looks good doesn't it? At a hospital.
No - it's not a very good reason. Just because you're not being paid doesn't mean you don't need a work permit. If you are doing a job unpaid that someone would normally be paid to do - then a work permit is still required. Any volunteer job in a hospital would very likely require a work permit and without a work permit - it will be classified as illegal work (even though you aren't being paid). So I would be very careful using volunteering as a reason for arriving early.
Okay. Thank you. Would it be better for me to say... I just want to vacation for three months with my family's friends/host family and get integrated into Canadian society? I do need to learn the metric system and celsius.
If you are from the US, you don't need to integrate into Canadian society - I would avoid saying that. I would also avoid mentioning the metric system and celsius (you don't have to be in Canada to learn these).