A passport is not always required for a bus trip to Canada, but a passport, passport card, or NEXUS trusted traveler card must be presented on entry into Canada. If you do not have a passport card or NEXUS card, then a passport is required.
Canadian law requires all visitors to Canada to provide proof of citizenship in their country of origin as well as proof of identity. A valid passport satisfies both of these requirements, as does the NEXUS trusted traveler card. For travelers under 16 years of age, only proof of citizenship, such as a birth certificate, is required.