Russia was very afraid that the British would try to take Alaska, and they knew they couldn't defend it - that's why they sold it to the Americans. It wouldn't be too difficult for the Americans not to purchase it - after all, general opinion in Washington was that it was little more than a cold, Arctic wasteland. Have the anti-Alaskan politicians be more successful, and Alaska quickly falls into British - and thus Canadian - hands.
Now, what will a Canadian Alaska look like, and what would it do for Canada? The well-settled Panhandle will most likely be split off from the rest of the colony. It could remain independent while being adjoined with British Columbia's northern territories (more likely; makes management easier), or it could become a part of BC. The rest of Alaska will probably become a part of the Northwest Territories. When the Yukon Territory is carved out during the Gold Rush, northern Alaska (meaning, everything that isn't part of the Panhandle) will probably be included with it. The Yukon becomes a province once Anchorage (or its analogue, whatever it's called) starts growing.
The town of Prince Rupert will probably be butterflied away, what with Sitka right nearby. Juneau, or some other town north of Sitka, may grow in importance the same way it did in OTL (Sitka was dependent on whaling and fur, and when those declined in importance, Sitka declined as well). Both towns will be a bit larger than in OTL (remember: no Prince Rupert), and there's a chance (as there always is in an ATL) that one of them will become a major city. Anchorage and the Mat-Su Valley will probably see development similar to OTL - the area makes a great east-west stop, especially for air traffic, and it's one of the few areas in Alaska that can really support agriculture.
With Alaska, Canada will be put into an enviable position, especially as time goes on. It will have all of Alaska's mineral resources (gold, oil), and it will have undisputed control over the entire northern third of the continent. This Canada is going to be richer and more powerful than in OTL, primarily at the expense of the United States. And when global warming picks up, Canada's undisputed control of the Northwest Passage (remember: no American claims in the region) is going to be a beautiful asset.