Puerto Rico and the U.S. Virgin Islands:
Puerto Rico and the U.S. Virgin Islands are a U.S. commonwealth and territory, respectively, so traveling to these islands is basicaly 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: you'll need a passport, Passport Card or other secure documents to cross over to the British Virgin Islands and then reenter the U.S. Virgin Islands.
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) face stiff 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.
A recently expanded exception is taking a so-called "people to people" tour to Cuba with a group authorized by the U.S. State Department. These tours are primarily cultural in nature, so there won't be a lot of beach time, but they do afford the average American to see Cuba legally for the first time in decades.
All other Caribbean destinations:
Generally require a valid passport for entry, and regardless, you'll need a passport (for all travel) or U.S. Passport Card (for land or sea crossings only) 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 Americans Traveling Abroad website.
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 sometimes 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 the Western Hemisphere Travel Initiative (WHTI), all air travelers returning to the U.S. from the Caribbean are required to present their passports at U.S. Customs.
Effective January 2009, WHTI required that adult U.S. and Canadian citizens arriving in the U.S. by sea or land from the Caribbean, Bermuda, Mexico or Canada present:
- a U.S. or Canadian passport;
- Trusted Traveler Card (NEXUS, SENTRI, or FAST/EXPRES);
- U.S. Passport Card; or
- State or province-issued Enhanced Driver’s License (when and where available)
Air travelers must have a passport; the Passport Card and other documents are not valid for air travel. Only children under age 16 will be permitted to travel with only a birth certificate or other proof of citizenship, although passports for children also are recommended.