Normally what you need at least if proof of onward travel. This means a reservation or ticket out of the country for a date that falls within your allowed stay.
The most common is a return airfare but can be any ticket in your name out of Canada, so even if you some from the US, you can have a ticket to fly to Iceland or India for that matter (anywhere outside of Canada). You are not limited to flights though which should be easy considering Canada and the US share the longest land border in the world (8893km). There are numerous trains and buses you can book in advance from Canada to the US.
Depending on your confidence in getting the visa, you can buy a normal airfare back but you can also opt for a fully refundable fare. There is always the chance of not getting the visa but this is how many countries work, even ones where getting a visa is not so easy. You may also want to get a refundable lodging, just in case.
When proof of onward travel is not enough, evidence that you hold a job can help. This will come as a letter from your employer saying that your are employed permanently or at least for a long contract. It may also state the day you intend to resume work after your holiday.