For a U.S. citizen in 2026, the only "international-style" destinations you can visit without a passport are U.S. Territories, which are not technically foreign countries but offer a distinct cultural and geographic experience. These include Puerto Rico, the U.S. Virgin Islands (St. Thomas, St. Croix, St. John), Guam, the Northern Mariana Islands, and American Samoa. Traveling to these locations is considered domestic travel; you only need a government-issued photo ID, such as a "REAL ID" compliant driver's license. Additionally, U.S. citizens can visit the Closed-Loop cruise destinations in the Caribbean, Mexico, and Canada without a passport, provided the ship departs from and returns to the same U.S. port. On these specific voyages, a state-certified birth certificate and a driver's license are legally sufficient for re-entry to the U.S. under the Western Hemisphere Travel Initiative. However, it is highly recommended to bring a passport anyway, as you cannot fly back to the U.S. from a foreign port in an emergency without one. For any other "true" foreign country, a valid passport remains an absolute legal requirement for entry.