The National Mall is easily accessible by Washington D.C.’s metro system. Use the Smithsonian (Blue, Orange, Silver) or Federal Triangle (Blue, Orange, Silver) metro stop to access the National Mall. A one-way fare is typically $2-3 per person. Some attractions, such as the U.S. Capitol, offer paid parking for personal vehicles.
If you have more time, consider adding a visit to the National Archives to see the original copies of the Declaration of Independence, the Constitution, and the Bill of Rights. If you have less time, consider skipping the U.S Capitol tour and Library of Congress and adding more time to explore the Smithsonian museums in the National Mall.