My friend Beth sent this suggestion to me, which I recommend heartily:
Walter Reed Hospital, Family Affairs, (202) 782-2071, confirmed that if you address a card as described below, your card WILL go to a soldier at the hospital. Your small act of kindness will be greatly appreciated.
Here's the address:
A Recovering American Soldier
c/o Walter Reed Army Medical Center
6900 Georgia Avenue, NW
Washington, DC 20307-5001