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If travelling by air, the only accepted document other than a passport is a NEXUS card, as stated in Timatic, the database used by airlines
Otherwise, as stated by the CBSA, you can also use a citizenship certificate, or Green card (not mentioned), or enhanced driving licence.
Or, alternatively, a birth certificate plus one of the following
• CANPASS card
• Commercial Driver Registration Program card
• Ordinary driving license
• Employment or student card with photo and signature
• Provincial Health Insurance Card
• Provincial identity card
• Canadian Forces Identification
• Police Identification
• Firearms Acquisition Certificate
• Social Insurance Number Card
• Credit Card
• Vehicle Insurance Certificate
• Vehicle Registration