What You Need to Know About Visiting Canada From the U.S.

Passport requirements have been a complicated and ever-changing issue for U.S. travelers to Canada due to the Western Hemisphere Travel Initiative (WHTI), which was introduced in 2004 by the U.S. government to strengthen U.S. border security and standardize travel documentation.
Visitors from any country other than the U.S. have always needed a passport to enter Canada. On the other hand, because of a friendly border crossing agreement between Canada and the United States, Canada Border Services did not require U.S. citizens to present a passport to enter Canada. This friendly border crossing agreement used to be mutual; however, now the WHTI requires that U.S. citizens have a passport to return home via air.

In this way, passport requirements for Canada and U.S. borders are different on paper, but, are in practice, the same. Canada will not allow a U.S. citizen into the country who does not have the proper documentation to return home.

The Canadian government requires citizens of the United States to have a passport to fly to or transit through a Canadian airport, but not to enter Canada by land or by boat. For those travelers, in lieu of a passport, Canada requires that you carry proof of your citizenship, such as a birth certificate, certificate of citizenship or naturalization, or a Certificate of Indian Status, as well as photo identification.

NEXUS Card

NEXUS is a voluntary program designed to speed up border crossings for low-risk, pre-approved travelers into Canada and the United States. The information you provide is used to determine your eligibility. 

NEXUS members use dedicated processing lanes at certain northern border crossing points, NEXUS kiosks when entering Canada by air and Global Entry kiosks when entering the United States via Canadian Pre-clearance airports. NEXUS members also receive expedited processing at marine reporting locations.

The U.S. Customs and Border Protection website has a list of trusted traveler programs including NEXUS and provides application information. 

Returning to the United States

U.S. citizens and permanent residents traveling to Canada by air are required by U.S. law to present a passport for entry into Canada and will need to present their passport again when returning to the U.S.
For those traveling by land or water, you will need a passport, passport card, NEXUS card, Enhanced Drivers License, or another Western Hemisphere Travel Initiative-compliant document to enter Canada, and will need to show these documents again to return to the U.S.

Children

Children under 16 need to present proof of U.S. citizenship when entering Canada with their parents. As with adults, the requirements are different for those flying. If minor children (under 18) are traveling alone, with one parent only, or with someone other than their parents, they may need additional documentation. This is for the protection of the children.

All children returning to the United States from Canada by air must hold a valid U.S. passport.

Recommendations 

Follow the most restrictive recommendations because you can't predict where your future travels will take you. Document requirements for travelers to and from Canada by air are required to have passports so even if you are traveling by car this time, consider obtaining a passport in anticipation of future travel plans.

Additional information, as well as current advisories, can be found on the websites of the Canada Border Services Agency and The US Department of State.