What do you need as a US citizen to cross into Canada and back again?
The problem is not entering Canada. It is returning to the U.S. Under the Western Hemisphere Travel Initiative (WHTI) U.S. citizens seeking admission to the U.S. must have either a passport, passport card, enhanced driver licence/ID or a NEXUS, FAST or SENTRI card. There are exceptions for persons 15 or under where they only need evidence of citizenship. If you try to enter Canada without WHTI compliant documents and declare you live in the U.S., you will be refused entry to Canada and sent back to the U.S. Get a passport.