I just decided to travel to Nova Scotia and I don't have time to get a passport. Documentary Requirements for Entry to the United States Travel from Mexico or Canada within the Western Hemisphere (North, Central or South America, Caribbean and Bermuda)
U.S. citizens:

Are currently exempt from passport requirements. However, it is highly recommended that a U.S. Citizen be prepared to present documentation such as valid or expired U.S. passport, U.S. birth certificate, or Naturalization certificate to avoid delays.

For further, more detailed information please visit www.customs.gov