Well, I'm in Illinois and every time I've gone to Canada I've flown, so I have no experience. More than likely, we'd be going north to Minnesota and try to go through at the easternmost crossing, then if that doesn't work we head west and keep trying. I mean, if the chances of getting in without a passport is merely "not 100%", I figure after a few tries we'll get to a more permissive/understanding/apathetic border agent, right?
And I think we'd be fine clearing customs on the way back. I mean, if we can't smuggle anything into the U.S., we don't have to.