Puerto Rico and the U.S. Virgin Islands: These are U.S. territories, so it's just like crossing a state border. No passport is required; if you are over age 18 you'll need an unexpired driver's license, state-issued photo ID, passport, or government employee ID; or two forms of non-photo ID, including at least one that has been issued by a state or federal agency. Note, however, that as of Jan. 23, 2007, you'll need a passport to cross over to the British Virgin Islands and then reenter the U.S. Virgin Islands.
Cuba: For most U.S. citizens, this is simple: it is illegal to travel to Cuba under federal law, and those who do (say, by taking a flight from Canada) faces still fines. A number of travelers have been caught coming back into the U.S. after a secret trip to Cuba by sharp-eyed U.S. Customs officials who noticed a Cuban customs stamp in their passport. Those who do travel to Cuba also need to obtain a visa from the Cuban government. For more information, see the U.S. State Department's website.
All other Caribbean destinations: Generally require a valid passport for entry, and regardless, you'll need a passport to get back into the U.S. Some countries also may require you to present a return airline ticket and/or proof that you have enough money to support yourself during your stay. The U.S. State Department spells out each country's entry and visa requirements in detail in its online Foreign Entry Requirements brochure.
More Advice: It's sometimes tempting to think of "the Caribbean" as a single entity, like "Canada" or even "Europe," but the truth is that the region is a polyglot of independent nations and territories that are politically linked to larger nations, including the U.S., France, Great Britain, and the Netherlands. Each has its own custom and entry requirements for visitors.
Under new rules that went into effect in January 2007, all air travelers returning to the U.S. from the Caribbean are required to present their passports at U.S. Customs; the requirement will be extended to those arriving by sea or other means by Jan. 1, 2008.
