For US Citizens -- birth certificate with a license for the picture or Passport.
Note that my sister was able to use her recently expired passport (she didn't check the date early enough to get the renewal) without any problems in January.
This is from the Passport Site regarding entry requirements
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">CANADA - Passport or proof of U.S. citizenship 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. For student or business travel, check with the Canadian Embassy, 501 Pennsylvania Ave., NW, Washington, DC 20001 (202/682-1740) or 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:
www.canadianembassy.org </font>