Campy,
Glad to hear about your trip to DC. Just a few completely random
thoughts/suggestions:
1. I'll echo what skid said about visiting the various museums of
the Smithsonian Institution. I've always maintained that every
human on Earth should, at least once in his or her lifetime, make a
"pilgrimage" to WDC for the express purpose of visiting the SI
museums. The wealth of exhibits and knowledge stored in them is
truly, truly sobering. Since coming here, I've visited WDC five
times just to tour the museums, and it never gets old. Hop over to
www.si.edu for more
information about the SI museums. Give yourself several days (I'd
even suggest an entire week) to hit all the major museums.
2. You might already know this, but downtown WDC is generally
not a safe place to be
after dark. The museums are open from 10:00 a.m. to 5:30 p.m., and
sunset during the intended time of your visit is around 6:00 p.m.,
so you should have enough
daylight left to retreat to the safety of the suburbs after the
museums close. 
3. The flight itinerary for your return shows you have 75 minutes
to make your connection at LHR for the flight to SIN. From past
experience, this is cutting it a bit close (I generally prefer 90
minutes or more for international connections), and there is a
slight but nonetheless real chance that any delay departing IAD
might cause you to miss the connection at LHR. However, if this
happens, I'm sure the airline will rebook you on the next available
flight (possibly even on another airline) to SIN. Worst case
scenario: you arrive at SIN a few hours late.
Have a great time there, and take lots of digipics!
