U.S. citizens can travel without a passport to the following destinations:

Puerto Rico
A U.S. commonwealth that doesn't require a passport or visa for U.S. citizens

U.S. Virgin Islands
A U.S. territory that doesn't require a passport for U.S. citizens visiting from the U.S. or Puerto Rico. However, you must show proof of U.S. citizenship and a valid government-issued ID.
Guam
A U.S. territory where U.S. citizens are considered domestic travelers and don't need a passport. You'll need a Real ID or a government-issued photo ID and an original or certified birth certificate.
American Samoa
A destination with pristine beaches and Polynesian heritage that doesn't require a passport. You'll need a government-issued photo ID.
Canada
You can travel from the U.S. to Canada without a passport if you cross the border by road or sea. You'll need a NEXUS card to prove your U.S. nationality.
Alaska
A U.S. federated territory that doesn't require a passport or visa to get into. You can travel by car, public bus, cruise, or direct flight.
Mexico
You can travel to Mexico without a passport or visa in some specific modes of travel, including by road, cruise, or direct flight.
Haiti
You can travel to Haiti without a passport or visa by ship, road, or barge car (ferry). You'll need a passport for direct flights, but not for domestic flights.

You can use a government-issued photo ID, such as a state-issued ID card, for domestic air travel.

4 Beautiful Caribbean Islands Worth Visiting—No U.S. Passport Required

Puerto Rico. Each year, Puerto Rico sees millions of visitors, and many head straight to San Juan, the island's capital. ...
St. Croix, U.S. Virgin Islands. ...
St. Thomas, U.S. Virgin Islands. ...
St. John, U.S. Virgin Islands.