Get your steps in while checking out the various memorials.
All Smithsonian museums are free! However, a few require timed entry passes, which you can reserve online: Air & Space (on the Mall but not the one by Dulles), African American, and National Zoo.
Free public tours are available, but must be coordinated AT LEAST 21 days in advance through your Member of Congress. Or just check it out from the outside.
Free public tours are available and can be booked online.
Free public tours are available and may be booked online.
Please note that D.C. is a pro-reservation city. While not required, if there is a particular cuisine or restaurant you want to go to, we highly recommend making a reservation (especially for larger parties).
There is never a shortage of things to do in and around D.C.