Before we had to have passports to get back into the States from Canada, we used our drivers licenses. Today, you can't get back into the States without the passport.
I was at a Canada/US border crossing gate being interviewed by the guard, ID out.
Guard: "Got anything to declare?"
Me: (I'm in a Geo Metro hatchback) "Nothing to declare, but I got a bag of M&Ms if you want some (I hold it up), and a broken water gun in back if you want it (clearly seen in the back)."
Guard: (laughs) "You may go."
That's probably the only time I've gotten a guard like that to laugh. These days, it could easily get you in trouble.