Primary Source: The Points Guy - https://thepointsguy.com/guide/places-americans-can-travel-internationally/
There are over 90 foreign countries and territories that Americans may visit right now. COVID-19 restrictions may include some or most of the following:
As of January 26th, all passengers arriving to the USA by plane must have proof of a negative COVID-19 test taken within 72 hours of departure to the US or a physician's certification of recovery from COVID-19. This requirement is for all passengers including American citizens and residents.
Some of the most popular destinations that are still off limits to Americans:
The ability to purchase a ticket for an off-limits country does not mean that you'll be allowed to board the plane from the US. Airlines are still flying to those countries so that residents can return home and to move cargo.
Travel restrictions change frequently due to COVID-19 so it is recommended that one keeps apprised of the latest requirements for their destination(s). Check with your destination country's immigration or consulate for specific requirements. Here are some good resources to get started: