Most Middle Eastern and African countries require a visa and passport to enter. Most do not require a return ticket but it is recommended for faster processing.
Western nations, such as Europe, Australia, Asia and the Americas do not require a visa to enter their borders by U.S. citizens in general. However, you do need to have a return flight ticket booked to show you will be leaving. If you wish to stay longer, or have an open-ended trip, you may need a visa.
Contact both your country's embassy and the embassy of the country you are traveling to so that you know the requirements. Stay aware of travel alerts, which can affect visa requirements. Most Middle Eastern countries will refuse entry if you have visited Israel even with all other required paperwork.