I once past the border with just my regular (non-enhanced) drivers license.
On the way into Canada they sent me inside where they asked me 2-3 questions and let me go.
On the way back the CBP officer just asked me "how do I know you're a US citizen?" And I answered "what do you mean? I was born in Brooklyn" so he said "next time please have your passport. Good night" and he let me go...