What identification should I take with me?

As per the Canada Border Services Agency Website:

Effective June 1, 2009 Canadian citizens are required to present one of the following valid WHTI-compliant documents when entering the United States by land or water:

• a passport;
• a NEXUS card;
• a Free and Secure Trade (FAST) card;
• an enhanced driver's license (EDL) or enhanced identification card (EIC) from a province where a U.S.-approved EDL/EIC program has been implemented; or
• a Secure Certificate of Indian Status (when this certificate is available and approved by the United States). In the interim, the current Certificate of Indian Status is an acceptable document to present when entering the United States by land or water.

U.S. entry requirements have not changed for Canadian citizens who are U.S. lawful permanent residents (LPRs). U.S. LPRs may continue to present their permanent resident card (Form I-551) or other valid evidence of permanent residence status in the United States.

Note: Canadian citizens entering the United States by land but exiting by air must present a valid passport or a NEXUS card in compliance with U.S. air exit requirements.

Canadian citizens aged 15 years or under are only required to present proof of Canadian citizenship, such as an original or a photocopy of a birth certificate, or an original citizenship card. Canadian citizens 18 years of age or under who are traveling with a school or other organized group, under adult supervision with parental/guardian consent, may also present proof of Canadian citizenship alone.

The United States has indicated that it will take a common sense approach to implementing the WHTI at its land and water ports of entry, which includes a period of flexibility after June 1, 2009. The Government of Canada cautions travelers that the length and extent of this flexibility is entirely at the discretion of U.S. officials.
U.S. entry requirements have not changed for non-Canadian citizens, including permanent residents residing in Canada.
For more information on border crossing requirements, please visit the Canada Border Service Agency website at www.cbsa.gc.ca