A Passport.........There is a new thing called PASSPORT CARD which is good for 5 years and is cheaper also!
Any child traveling with an adult whom is not his or her parent needs a letter of permission from the parent.
PROOF OF CAR INSURANCE-
The owner of the car that is crossing the border also needs proof that the car is insured in Canada.  This is obtained by going to your USA insurance office and asking them if it is covered.  If it is then they give you a form or card to prove it when you get to the border.  They dont always ask for this but it is necessary if they do decide to ask.