The feds have an office at Harbourfront that's open 7 days a week. Was able to apply for my daughters SIN number yesterday (Sunday). Apparantly its the only federal office open 7 days a week across the country.
It took all of five minutes. There was nobody there but two employees.
Address is 235 Queens Quay W (IIRC). Its a small standalone building just west of Queen's Quay Terminal. Looks like a tourist info building (which it is beyond being a proper federal office).