Another local anime convention, AUSA will be metro accessible this year - and right next to the zoo!
Its mission is to educate the public about and promote Japanese arts and popular culture. AUSA is a convention of fans, by fans, and for fans. For more, see http://animeusa.org/
Registration is $45 for the weekend. They normally sell day passes at the door as well.
Discussion thread is at: http://www.meetup.com/Bronies-DC/messages/boards/thread/23752532