From the United States
If you are travelling to Canada from the United States and you are not a permanent resident of either country you need to be careful to satisfy the U.S. authorities on any subsequent trip that you have not exceeded their limits on stays in North America. Your time in Canada counts towards your maximum allowed United States stay if you are returning to the U.S. prior to your departure from North America.
If you are returning to the U.S. in this trip, keep your visa documents. Do not hand over your US visa or visa waiver card (I-94 or I-94W) to border control. You can enter the U.S. multiple times during the time allocated to your visa (for Western tourists, normally 90 days), but you need to have the immigration document as well to validate the visa. If you come back from the U.S. without that document, you will not only have to apply again for a visa or visa waiver but also will also need to satisfy U.S. immigration of the validity of your trip (meaning to show them that you will not intend on immigrating there).
If your default U.S. time is going to run out while you are in Canada, and you want to return to the US direct from Canada, you need to apply for a U.S. visa with a longer time period (eg B-1/B-2, or a C-1 transit visa) before your first trip through the U.S.. For example, if you are going to stay in Canada for six months, and you transit through the US on a visa waiver, then the U.S. will regard your six months in Canada as not allowing you to return to the U.S. without leaving North America first, as you have stayed more than 90 days in North America in total. Note that in this scenario, you have not done anything wrong by visiting the U.S. and then staying in Canada for a long time, simply that the U.S. will not allow you to return directly from Canada, you have to reset their clock by leaving North America. Visa waiver travelers may be able to avoid this by returning their I-94W (green) form to their airline upon departing the U.S., or to the Canadian immigration inspector if entering Canada by land; since the U.S. has no outbound immigration check, it's up to the traveler to remember this.
If you are intending to leave North America entirely without returning to the U.S. on this trip, return any visa documents at the time of leaving the U.S. for Canada. This means handing over your I-94 or I-94W card to airline staff at the check-in counter if departing by air, or to the Canadian immigration inspector if departing by land. If you do not, you will need to prove [7] to the U.S. that you didn't overstay to be admitted on future trips (the US CBP[8] website has information on how to correct this mistake).
If you leave Canada to briefly visit the United States and wish to re-enter Canada in a short period of time, you generally may do so without getting a new Canadian visa as long as you return within the initial period authorised by the immigration officer or have a valid temporary residence permit authorising you to re-enter, and you do not leave US soil before returning to Canada (i.e. not even during a cruise which begins and ends at a US point but crosses international waters in-between). If you leave US soil for a third country for any reason on a single-entry Canadian visa, you will have to apply for a new visa before re-entering Canada.
[edit] By plane
You are likely to arrive to Canada by air, most likely into Montreal, Ottawa, Toronto, Calgary or Vancouver (the 5 largest cities, from East to West). Many other cities have international airports as well, with the following being of particular use to visitors: Halifax, St. John's, Winnipeg, Edmonton, Regina, Saskatoon, Kelowna, and Victoria.
