Getting around the US capital is designed to be easy and Washington, DC boasts one of the most functional transport systems in the world. The underground MetroRail allows you to quickly get from place to place, with the MetroBus then servicing any additional stops.
If you’re looking to really explore the monuments and museums then walking is the best way to take in the full scope of your surroundings. The majority of the city’s most famous sights are all accessible on foot from the central point of the National Mall.