Spring break time is as bad as summer for the crowds, so expect lines. You have to have bags checked at most buildings and some don’t allow food or water, even unopened.
The Postal Museum is next to Union Station, not the Library of Congress.
The dinosaur exhibit is closed for renovation, as is much of the Hirschorn and half of the American History. So are most of the galleries of the West Building of the American Art Museum.
The National Cathedral now charges if you go inside, except on Sundays.
Ford’s Theater is open only by tour and taking the tour was the only way to see the Peterson house. You can make reservations online.
You need tickets to go up the Washington Monument and can get them ahead of time via the National Parks webpage for a fee. Otherwise you have to be in line very early in the morning and beat the tour operators.
You also need tickets for the Bureau of Engraving & Printing and I believe you will for the Holocaust Museum at that time of year.
You need tour tickets for the Capitol building, except for the exhibits in the Visitor Center. You can get them at the Visitor Center or via your congressman.
White House tickets are available through your congressman on a limited basis and probably long gone for that week.
Lines at the Archives may be long, so be prepared for that.
You can go through the FDR Memorial while walking to/from the Jefferson Memorial and hit the LIncoln, Vietnam, Korean and MLK at the same time. The WW2 one is by the Washington Monument.
There are underground walkways between the two American Art museums, between the Capital and Supreme Court, and between the Freer Art Gallery and the gazebo outside it. There are underground galleries there.
There isn’t much in the Castle but it was the original building and worth a walk through.
Don’t try to drive down and park unless you know exactly where you can find parking. Meters are two hours and it’s a pain to have to run back and forth to your car. I live in the area and often park at the Pentagon City mall and ride a couple stops in.
Have fun!