Plan an entire day for the Holocaust Museum. I've gone twice and still have not seen it all.
The museum is an intensely educational experience, walking you through the experiences leading up to the Holocaust, and the Holocaust itself. The main exhibit is a mostly chronological walk through the rise of the Nazi Party and of Adolph Hitler, the roots of and rise of anti-semitism in Germany during the first half of the twentieth century, and leading you through increasing limitations on the lives of the Jewish inhabitants of Germany and German-controlled lands, and the emprisonment, torture, and mass murder of six million Jews and others.
Some of the "highlights" of the museum, for me, included the passport that they give you, which tells you the story of one person as you walk through the exhibit - you read what happens to this person. It really personalizes it for you. Another really affecting thing is the tower of photographs, which were taken from a town in Poland that was completely wiped out by Nazis during the Holocaust. A reproduction of a beautiful synagogue, what it looked like before and after Krystelnacht.
But the most moving thing for me is a glassed-off room on one floor, which you enter by stepping over a "street" paved with gravestones from a Jewish cemetary, where you can sit and just listen to the recorded voices of Auschwitz survivors telling their stories.
This is not a comfortable, fun museum, but it is an important one. I have since been to Buchenwald and Auschwitz (and have written them into journals here) and being there, standing on that ground, was moving, very moving, but this memorial seems to have more to teach.
There is often a special exhibit or exhibits available.
There is also the memorial itself, on the ground floor. I recommend that you see the exhibits before you spend time in the quiet meditative area of the memorial.
Be advised that you have to get tickets to that museum. You can get them at the ticket window in the morning of the day you want to go. It is harder to get them on weekends and in summer or during Cherry Blossom time. Another option is to get them in advance through Ticketmaster. Although the tickets are free, Ticketmaster does charge a handling fee.
This museum is much too intense for children under 12, but they do have a gentler exhibit following the life of one boy through the experience, specifically for children. I don't think you need tickets for that.
The closest metro is Smithsonian, on the orange and blue lines, but don't exit the station toward the mall, take the other exit. The Holocaust Memorial is next door to the Bureau of Engraving and printing.
Their web site is at ushmm.org.