I traveled monthly to British Columbia in 1996. I took my passport. One day, I forgot it. Getting into Canada was easy. Getting back into the US was like pulling teeth. The customs agent went on this big lecture trip (for at least 10 minutes), on why I needed a passport for proof of citizenship. This was after I warned him I had forgot to bring it on the flight up the day before. Needless to say, I always take it with me.