The U.S. Postal Service is a surprisingly good source for no-passport-required research.
On the agency’s Web site, click to the page where you ask to have your mail held. Pull down the “State” tab and scan the alphabetical list that follows “Arkansas.” You’ll see “American Samoa,” “Federated States of Micronesia,” “Guam,” “Mariana Islands (Saipan, Rota)” and “Palau” intermingled with our 50 states.
Use this information for inspiration only; don’t book a flight based on USPS mail delivery. The legislative intricacies and arrangements between the U.S. government and these locales are tricky and determine the travel requirements for U.S. citizens. For example, Palau, the Marshall Islands and the Federated States of Micronesia share political ties with our government but are sovereign states. This means that I would need a passport to visit the islands and to re-enter the States, because our government regards the islands as foreign destinations.