Travelers from every country who come to Canada by air, land and sea needs a passport or equivalent travel document. When you are returning to the US from Canada, it is mandatory that all US citizens present a valid US passport to enter or re-enter the US through air. While entering the US through land and sea borders, US citizens have to show either a US passport, passport card, NEXUS card, Enhanced Drivers License or other Western Hemisphere Travel Initiative (WHTI)-compliant document.
US citizens under the age of 16 (or under 19, if traveling with a school, religious, or other youth group) are exempted from this requirement. They just have to present a birth certificate (original, photocopy or certified copy), Consular Report of Birth Abroad, or naturalization certificate.
Americans can enter Canada with a government issued ID and birth certificate but they alone are not good enough to get them back to the US. You should have a valid passport to enter the US. Make sure you take take all of your ID along with you including your passport to guarantee re-entry back into the US. Since 9/11, car searches have become very common.
Children under age 16 will be able to continue crossing land and sea borders using only a U.S. birth certificate (or other evidence of US citizenship such as a naturalization certificate.). The original birth certificate or a copy can also be used. If you are traveling with children and entering into Canada or the US, you should have identification for each child. If you are a divorced parent sharing custody of your children, you should have copies of the legal custody documents. If you are an adult who is not the parent or guardian, you should have written permission from the parents or guardians to supervise the children.
Dogs and cats from the US that are at least three months old need a signed and dated certificate from a veterinarian certifying that they have been vaccinated against rabies within the last three years. The certificate should identify the animal clearly. If you are bringing you dog or cat that is less than three months old, you do not need a certificate of rabies vaccination to enter Canada. But it is important that the animals are in good health when they arrive. Most pet foods are not allowed to cross the border.
NEXUS is a highway program (joint Canada-US initiative) that allows low-risk, frequent travelers to cross the border through dedicated lanes.