Check out the two boxes in Sugarland Run Stream Valley Park at

http://www.mindspring.com/~alberth/lbna/letterboxes.html#cake

Clues from the web site:

Sugarland Run Stream Valley Park is a long, thin park surrounding the
small creek that runs through it. Get on Route 7 and take to Route
7100, Fairfax County Parkway. Head South and turn Right at the first
road onto Sugarland Road (route 604). Take this all the way to the
end and park in the turn-around.


A Slice of Cake
---------------

There are two paths from here. A wide one that comes right up to the
road, and another, thinner bike trail on the left up the hill
slightly. Walk to the bike path and head right. Left takes you a
short distance to the actual start of the path about 300' behind you.
You are on the correct path if a pond appears on your left. At the
fork, go Left. You will pass a basketball court, the path with make
an "S" turn, then turn sharply right. Continue to a small stream with
stepping stones crossing it. Cross and continue a short distance to
where the path zig zags slightly with a tree that forks at its base,
immediately off the path on your right.