You're leaving the United States, so pack your passport, right? Not always. Some countries don't require a passport, even though they're technically foreign destinations. (It never hurts to have your passport handy, though, to make border crossings or getting through customs easier.)
The areas in question are Canada, the Caribbean and Mexico.
* Canada: Proof of citizenship is required, including a government-issued birth certificate, certificate of citizenship or naturalization certificate, as well as a valid photo ID, such as a driver's license.
* Caribbean: The following islands require only an original or official birth certificate and a valid driver's license or official picture ID: Anguilla, Antigua, Aruba, the Bahamas, Bermuda, Bonaire, Curacao, Puerto Rico, Montserrat, St. Maarten/St. Martin, the British and U.S. Virgin Islands, Dominican Republic, Dominica, St. Kitts, Nevis, St. Lucia, St. Vincent and the Grenadines, Granada, the French Government Islands (St. Barts, St. Martin, Guadeloupe, Martinique), Jamaica, Turks and Caicos, and St. Eustatius.
Barbados, Belize, Trinidad and Tobago do require a passport. For other islands, contact the Caribbean Tourism Organization (212-625-9530, www.doitcaribbean.com) or the tourism office of a specific island.
* Mexico: For all parts of the country, an original birth certificate and a valid photo ID will suffice. Driving to Tijuana, you can cross the border with only a U.S. driver's license, but Mexico's tourism office suggests bringing your passport and/or birth certificate -- just in case.