If you request for an appointment, they will call you in 2 business days however some cases have gone past the 2 days. You don't have the luxury to choose appointment time and date online. So for someone only here for a few days...wouldn't work.
If you apply online, you will need an address for them to mail your SIN. Again wouldn't work for me. I can give them my niece's address but there are many cases of identity theft in North America and UK. Something we rarely, infact I have never seen or heard of a case in Australia. My concern is not my niece, it's the fear of the mail man/woman either locally(when service canada mail my SIN to my niece) or internationally(when my niece posts to me) opening my mail to check contents. For PR card I don't mind, I can easily replace plus difficult for someone to use but SIN? I can't be in Australia minding my business only to find out someone has been flexing with my SIN in Canada or I might not even find out for a long time. It will be a nightmare fighting it when you are not in the country.
Documents used for my walk in was passport and COPR original. They will give you a slip/form and ask you to complete. Questions in the form includes your parents names, your address(i don't have a permanent address but the customer sevice guy advised I put down my temporary address, so I used my hotel address), phone number (advised to leave blank as I do not have a Canadian number). He asked for my email address and emailed me some info re protecting your SIN. Handed me a piece of paper with my SIN and I was on my way.
I got there at 9 and left at 11:16. My advise if you are doing walk in is get there at 8.30am. The queue was still crazy the second time but I got there a bit early which worked out fine.