[PSUSMU]

I'd skip Six Flags - the crowds/people there are disgusting.
While in DC, check out the White House obviously (front and back, most people don't see the back which is much closer). If you want to go up in the Washington Monument, you have to show up early in the morning to get tickets. Vietnam, WWII, and Lincoln memorials are nearby. Smithsonian museums, Holocaust museum, Newseum, etc. You can tour the US Capitol also.
Ben's Chili Bowl is the go-to food place, but nearby Meridian Hill Park aka Malcolm X park is cool if you're there for the Sunday afternoon drum circle.