To the original poster..........
Remember that as of July of 2009, the US Government requires every person, including US citizens, to present a valid passport, to enter, or re-enter , the USA.
As a result of that American law, Canada ALSO wants to see your passport, or an enhanced driver's license, from one of the few US states that issue them. Or a Nexus card.
Nexus is a program for those that are "frequent border crossers ' and it requires an application and a back ground and criminal records check, by both Canada and the USA, to get one. It is unlikely that a ordinary tourist will have the need for a Nexus card.
A further reminder, to my American friends, that intend to enter Canada..........We have very strict firearms regulations here, and they are national, meaning that the exact same laws apply in all parts of Canada. Rifles and shotguns CAN be imported, but it requires that you meet specific regulations and are prepared when arriving at the port of entry, to fill out the paperwork, have your weapons identified, and then pay the cost of the permit. In addition they must be unloaded, and the ammunition must be secured, in a separate lock box. Oversize rifle magazines are not allowed in Canada, as are Teflon tipped ammunition. We call those "cop killers ".
Hand guns are prohibited in Canada, and very few Canadians have either the need or the legal right to have one. Even our Canadian Police officers are NOT armed, when off duty. So, be aware, and obey our laws while in our country.
Jim B.
Toronto.