I answered passport as that is the instruction that was issued by the Postmaster here. There was an article in the Minneapolis-Star Tribune about relaxing the passport requirement for the Summer, posted June 29, 2007 -
Passport problems may go from bad to worse
This is from the website, "Foregin Entry Requirements" :
" Passport or proof of U.S. citizenship such as a birth certificate, naturalization certificate, and photo ID required. Minors (under 16) traveling alone or in someone else's custody must present written authorization, signed before a notary, from the parent(s) or guardian. Visas are not required. Canadian immigration officials at ports of entry will issue persons planning to stay longer than 180 days a visitor's record. Anyone with a criminal record (including a DWI charge) should contact the Canadian Embassy or nearest Consulate General before travel. U.S. citizens entering Canada from a third country must have a valid passport. HIV testing required for resident applicants who are over 15. A temporary resident applicant requires immigration medical examination if stay exceeds 6 months or seeking to work in Canada. For student or business travel, check with the Canadian Embassy, 501 Pennsylvania Ave. NW, Washington, DC 20001 (202/682-1740) or the nearest Consulate General: CA (213/346-2701), MI (313/567-2085), NY (212/596-1700 or 716/858-9501), or WA (206/443-1375). Internet:
Embassy Washington / Ambassade du Canada à Washington
From the Canda Border Services Agency, "When you enter Canada, a border services officer may ask to see your passport and a valid visa, when necessary. If you are a citizen of the United States, you do not need a passport to enter Canada; however, you should carry proof of your citizenship such as a birth certificate, certificate of citizenship or naturalization or a Certificate of Indian Status, as well as a photo ID. If you are a permanent resident of the United States, you must bring your permanent resident card (i.e. green card) with you. For more information on admissibility into Canada, read the fact sheet called Managing Access to Canada. It is available on our Web site at www.cbsa.gc.ca."