The one, and probably only thing, Canada and Mexico could do is put in place countering tariffs on goods imported from the U.S.

If the U.S. truly follows through with pulling out of NAFTA and putting in place a whole bunch of tariffs, likely not just on Canadian and Mexican goods but also on goods from China, Japan, Europe, etc., it would be all but certain to trigger a fairly large global reaction of tariffs.

That is exactly what happened in 1929/30, the U.S. put in place a bunch of tariffs to "bring jobs home" and Canada and Europe responded in kind.  The result was that some jobs came home as imports declined, but FAR more jobs were lost as exports plummeted.  That trade war was a key cause of The Great Depression, and the impact of similar measures today would probably be much worse.

Such a move would badly hurt Canada's economy but it would also decimate the U.S. economy.