Since it's the middle of the summer, museums are the way to go.
The National Museum of African American History and Culture is amazing. I recommend starting in the basement (where the exhibits chronicle the struggle past and present of the African American Experience) and then work your way up where the top floors celebrate the cultural contributions of African Americans (sports, music, politics, art, etc). The basement is really the only "must visit" floor in the museum (the Trayvon Martin exhibit hit hard, but was curated excellently), as it's a massive museum and would take hours to see everything. It's also in a good location on the mall too, close to Washington Monument and the White House, so it could be a good hot weather respite in the middle of the day.