Passport is BEST.
You can show your green card and have your drivers license. just to take extra precautions you can contact the u.s. embassy or consular services.
A passport is not mandatory for entry into Canada but it is a preferable piece of identification. Other acceptable identification includes an enhanced driver's licence, NEXUS card, a Free and Secure Trade (FAST) card, a birth certificate with accompanying photo I.D. such as a regular driver's licence, a permanent residence card, a citizenship card or a certificate of Indian Status.
Keep in mind that the Western Hemisphere Travel Initiative (WHTI) is now in effect. This is a U.S. law that requires all travellers to the United States to present a valid passport or another approved secure document at the border. The following forms of identification are acceptable for entry into Canada and the U.S. and meet the requirements of the WHTI: a passport, a NEXUS card, a FAST card and an enhanced driver's licence.