The Loon should be Canada's official national bird. It kind of already is, backing up our coinage.. like the bald eagle does in the U.S. That eerie call of their's that i heard all the time when on canoeing trips to Algonquin gives it a mystical presence in wild places.. like no other bird.
As for a national flower, the problem is it is hard to find one flower that symbolizes such a diverse biological area. The plants of the northern tundra are unique and grow no where else, the same for each of the botanical regions of Canada. I like the Trillium or Wild Rose.. but both those are taken by provinces.
I guess i'd nominate the Pine Cone.. It's a seed pod (i think), not really a flower, but i don't think you need to draw the definition too tightly. Scotland uses the thistle as its symbol, and its not a flower. The Pine Cone is everwhere in Canada, has a nice rustic appearance, and always appears to be hunkered down to the survive the winter, before setting down its roots.