You can currently enter Canada without a passport... but you cannot reenter the US from Canada legally without one- the exception is if you are an illegal alien crossing the southern border. Absurd reality. I’ve applied for permits to enter Canada the last three years in a row at monument marker # 78. That is where the Pacific Crest National Scenic Trail terminus is, and the location is not staffed by either US or Canadian border security agents. The trail continues on for 8 miles into Canada, leading to Manning Provincial Park. Because the crossing point is not staffed, US law prohibits reentry at this point on our northern border.
That's what I ran into in 2000. Throughout my life I made a number of fishing trips to Canada and all you needed going both ways was a drivers license. The last previous trip was 1985 and no passport was needed. I had been into Mexico in the mid nineties with out.
In 2000, the gal at the airport asked me if I had a passport, which I didn't. She told me I could probably talk my way into Canada but I would need one to return. In lieu of that I went home and got the birth certificate and caught the last flight out.
No problem getting in to Canada and on the return the US border inspection station was closed, so I didn't need anything.