About sixteen years ago (I remember, because it was back before 9/11, when you could go to Canada without a passport) I got bored one weekend and drove up to Vancouver by myself. I got to the border, and answered the standard questions like "who do you know here" and "is this your car". Then the border agent asked me the purpose of my visit, and I said "uhhhh, tourism, ha ha". They let me in about an hour later after the car search and police background check came up clean. I guess the moral of the story is come prepared and try to be convincing, because they can screw you over on a whim.

On the other hand, I've gotten across with no delay at all by saying "I'm picking up a friend at the Vancouver airport" and "I'm going to go buy this drill press off of Craigslist -- see this printout?" Neither of those is illegal, and in neither case can they expect you to have hotel reservations or know any Canadians, etc.