Until recently in Canadian history, Americans have been the largest immigrant group. They began arriving almost immediately after our revolution, too. During and after the War of 1812, many Americans were suspect, and many were hanged--quite a few in no better than a lynch mob mentality. The same thing happened in the uprisings of 1836-38. A lot of Canadians went the other way, too. Despite any rough spots in the road, Canadians and Americans both profited from opportunities in one another's countries.
Most things are similar between the United States and Canada, the differences are minimal. Many of those differences, those which aren't glaringly different (such as in the "ethnic" communities--Chinese, Vietnamese, West Indian, Indian, Pakistani . . . you know, non-European--which you have in some American cities, too) come from the "Englishness" of Canada. You can buy crumpets in the supermarkets, bangers, tea cakes, a gazillion kinds of tea (although they use tea bags, just as the Americans do)--but you also see items and brands familiar from the United States, that you wouldn't necessarily see the rest of the English-speaking world. The automobiles on the streets are just about identical--the "big three" brands of General Motors, Ford and Chrysler, as well as the German and Japanese competition. The chains of gas stations are completely different, except for Flying J, a highway-type fuel center for passenger cars and transports, which is also found in the U.S.
And some words are slightly different, too. Canadians go to a gas bar for the same purpose that Americans go to a gas station--to fill the tank with what i believe you call petrol. What Canadians and most of the English-speaking world call transports, Americans call "Semis" (from semi-tractor trailer)--although "truck" is used in both countries, rather than lorry. Canadians also can't properly pronounce the word schedule, and chronically misspell words such as center, color, humor, etc. Some of their pronunciations are unique Canadian, though. When Canadians (or at least people from Ontario) say about, or house, it's noticeably different, although i don't think i could say it exactly as they do. Older, "English" Canadians (i.e., of European descent even if not actually English) in Ontario make a two syllable word of such words as known, shown, grown--which they pronounce know-wen, show-wen, grow-en; and, many of them make a two syllable word of iron, which they pronounce eye-run, and which Americans pronounce as one syllable: eyern.
The metric system is a slight difference, although there it gets hilarious sometimes. At the gas bar, you buy anti-freeze or windshield wiper fluid in four liter containers, which is a close to a gallon as makes no difference. Although the package may be labeled 454 grams, they are still selling butter by the pound. If you walk into a supermarket, go to the deli counter and ask for pound of ham, that's what you'll get. I have yet to encounter a Canadian, no matter how young, who didn't know what i meant by that. There is a chain of butcher shops call M & M Meats, who don't quibble--they offer their products weighed and marked in pounds and ounces. Even when marked in metric measurements, many items are just the same size as they are in the United States when measured in pounds or ounces. If you go to the "home center" and start talking inches and yards, they know what you mean--although this is one area in which you might get a blank look from younger Canadians.
The metric system seems to have taken most in distance measurement and temperatures. You'd have to convert miles before most Canadians (except for the older ones) knew what the hell you meant. And they use centigrade for temperature measurement, so habitually that i've had even middle aged Canadians give a look like i'm a shameless liar if i talk about temperatures in the 60s--when you start talking about temps in the 80s or 90s, though, it seems to click for them and they understand that you're talking about that other system of measurement. You'd have to convert it for most of them to understand it.
The last big difference is that Canadians are unfailingly polite--and really snarky if you get under their skin. For example, a guy came to the door once representing himself as a representative of a firm of lawyers (you know, solicitors) working for the bank which holds the mortgage (it was all a misunderstanding, as it turned out). I asked him for identification (don't try to get in the door, or even talk to me unless you produce ID), and he got upset. He finally left at a point at which it might have gotten really acrimonious between two Americans, but he wished me a good day--although in a truly hostile tone of voice. He remained courteous, but only in a "technical," superficial way. He was boiling mad (which i rather enjoyed).
There is a motion picture which the late comedian John Candy (a Canadian) made, called Canadian Bacon
. He is one of the protagonist, playing a small town Sheriff from the United States. In one scene, he and his cronies are pushing through a crowd in Montreal. They are shoving people out of the way, even knocking some of them down. All the Canadians they are shoving or knocking down are muttering things such as "Excuse me," "Oh, sorry" (they pronounce "sorry" differently than we do, too)--which is a joke only Canadians and a handful of Americans would very likely get.