NAFTA ending appears to be thought of as free trade ending. This may be true in regards Mexico, however it's very untrue in the case of Canada and the USA. Canada and the US's ties predate NAFTA and to fully end free trade between Canada and the US would take more than just NAFTA being repealed.
First it needs to be noted how integrated Canada's economy is with the US, especially within manufacturing. Even a 'made in America' car will see pieces built in Canada (Ford makes its fuel pumps almost exclusively in Canada)...to repeal the concept of free trade is pretty close to ludicrous and impossible. To go a bit further, most of Western Canada is on the same power grid as California and there are plenty of times where a home owner in Los Angeles can flick on a light and use the same power that's currently running a computer in Vancouver. Cross border shopping drives a good chunk of Washington state's exports. I don't think you could feasibly end free trade between the two nations any easier than New York could end free trade with Texas.
So what does a NAFTA repeal mean to Canada and the US? There is a 1989 agreement that was never repealed between the two nations that defers to NAFTA currently and would take its place if NAFTA was repealed. What NAFTA does is set up the regulatory format in which disputes are handled. Under NAFTA, this heavily favors the US and is why Canada would favor the scrapping of NAFTA.
There are a few disputes along the lines of dairy (Canada is protectionist of its dairy industry from province to province, let alone internationally), other farm products (including eggs), and softwood lumber. Currently the US can basically dictate to Canada what it feels and Canada has little recourse...as we are seeing Trump make use of now.
More-over, there is what is known as the "proportionality clause" which guarantees Canada send a certain amount of its oil production to the United States (primarily flowing into Texas out of Alberta). If Canada was to hit an energy crisis, under NAFTA they would have to continue sending oil to America regardless of their own needs. In 1989, this was a decently big thing as it protected American energy needs in the case of a nuclear war with Russia. In modern days, it's the key agreement that keeps Canadian oil on a discounted market and forces them to sell to the US regardless of more lucrative Asian markets. With NAFTA gone, there is little to prevent Canada from redirecting its oil through projects such as the 'Northern Gateway' to international markets. The end result is a near 10% instant increase in gasoline prices across the US as the cheap oil NAFTA guarantees is no longer guaranteed...Canada can use its oil as power leverage in all trade disputes with the US.
The major change with a NAFTA repeal has nothing to do with free trade between Canada and the US...it has everything to do with negotiation and leverage in trade disputes. NAFTA heavily favored the US...now Canada can insist on full prices as per the international oil market every time the US tries to address Canadian protectionism.
More recently...Bombardier got into some issues that saw a 220% tax levied against them. Under NAFTA, the US has this leverage. Without NAFTA, Canada gains a significant amount back.
Edit a bit more:
It's easy to call Canada the small trading partner here, but it's important to note how incredibly close the relation is and ultimately how large it is. It's easy to find claims that US-Mexico trade 'exploded' to $295 billion in 2016, a look into canada - us numbers by the the US embassy in Canada shows how small the us-mexico relation is. Also interesting, given our question posters disclaimer, that Canadian companies pay American employees some of the highest wages in the US.
U.S.-Canada two-way trade
in goods and services totaled
nearly $759 billion in 2014.
U.S. and Canadian bilateral
investment stock totaled
nearly $698 billion.
U.S. exports to Canada totaled $375 billion in
2014 – 16 percent of total U.S. exports. Canada
is the number one export market for 35 U.S.
Canada and the United States trade more than $2
billion in goods and services daily.
U.S. exports to Canada exceeded total U.S. exports
to China, Japan, South Korea and Singapore
combined in 2014.
U.S. subsidiaries of Canadian firms employed more
than 546,000 employees in 2011, for an average
wage of over $65,000 annually.
Compare these numbers with mexico around $550 billion, around 2/3rds of Canadian US trade...completely neglecting the foreign investment involved. sourced: https://www.census.gov/foreign-trade/balance/c2010.html