You should apply for F1-OPT during the grace period. It takes 450$ for an application. I contacted my Office of International and requested for the new I-20 OPT, and it takes only 4 working days to have it issued (though I do not know about ur school, it may depend). Anyway, you can be legally present in US while waiting for the final decision for OPT. And one thing that I have to reveal you is that the time processing for I-20 OPT is now similar to waiting for study permit (taking forever....). I have applied since April, and now it is still pending (Good Sign!!!). So you should take advantage of having it pending to be legally in the US while waiting for ur Canada visa.