go to the black cat nightclub....
http://www.blackcatdc.com/
..very cool..

some of my favorite museums:

national building museum link
international spy museum link(advance tickets needed)
hirshorn museum link (best of the smithsonian, imo)

-union station is a good starting point
-check out the interior of the jefferson building at the library of congress
-stroll the capitol grounds but don't waste the time on a tour unless you like 19th century interiors, statues of dead guys, and/or being close to power
-take in as many museums as you can on the mall. i wouldn't miss the hirshorn or the national gallery and it's sculpture garden. also the us botanic garden on the mall is overlooked but pretty cool.
-when you're on the mall, hop over to 11th & pennsylvania ave and have lunch at the old post office pavillion. very cool old building that survived in the midst of some horrible 1930's neoclassicism. plus the food is better and cheaper than the museum cafes.
-check out the lincoln, jefferson, and vietnam memorials at the far end of the mall.
-dupont circle and adams morgan are the neighborhoods where you want to spend your evenings. dupont is a good place to peoplewatch, and a good place to have drinks or coffee. adams morgan has the true nightlife.

if you're hoping to see some nice modern architecture...prepare to be disappointed.

however...the New-seum by Polshek and Partners should be finished by now. (museum of the news and media). He presented it and other projects at a UF lecture a year or so ago, and it looked rather interesting, especially considering the siting of it.

I 2nd the memorial walk...the weather should be really nice this time of year too...

I was at the site of the Newseum, its got quite a bit of work left before its finished.

To see + do in DC;

Architecture/Design:
I.M. Pei's East Wing, Its awesome. There's a Richard Serra there and another about 2 miles away in the Holocaust Museum.
Check out the FDR memorial, I'd recommend going after dusk, the lights at night are incredible.
Hirshorn Museum, its across the Mall from the I.M. Pei building, its a big cylinder.
Next to that is the recently completed Native American Museum. The outside is wild, the interior resembles the NY Guggenheim. Worth a visit.
The Korean War Memorial is interesting too, get there just before sunset.

Party/Nightlife:
The big clubs are Fur and H20. Most architects seem to be introverted so I assume you're want to hit some chill bars so go to Georgetown or Adam's Morgan. They're not far from each other.
Georgetown is a mile-or-so long strip of shops, bars, etc, and its busy throughout the night. Parking sucks, if you go before 9:30/10pm you'll have a much better chance of finding a spot.
Adam's Morgan is more the young-professional's club spot. G-town is more bars.

If you have a long weekend or a week, make a trip to old-town Alexandria. Just across the bridge southwest of DC.

That's all I can think of for now.

Oh forgot:
There's an exhibit at Arlington National Cemetary done by CORE Architects in DC, recently completed, very cool. It's sad to see that many faces of recently killed soldiers, but I was impressed with the design of the structure.
And hit the National Building Museum, its a quick walk north from the mall, I think they have a liquid stone(concrete) exhibit going on still that is pretty interesting. The translucent concrete is worth playing with.

on the more non-architectural . . .
second vote for black cat. awesome place even if you just want to play some pick up pool or sit at a bar. the red room is free. also, around there . . . cafe st ex (14th and t)
check out u street for great clubs and historic buildings. dc9 is a great bar.
if you head to adams morgan, pharmacy bar had a great jukebox and good tap selection, amsterdam falafe has the best fries and falafel. open late which is rare in dc. bossa, rhumba cafe and the reef are all cool bars. buffalo billiards for more pool, and more beer. tryst has excellent coffee and is open early till late. the diner next door is where i used to work, and i can honestly say i will eat grilled cheese nowhere else. also great salvadorean bakery on columbia rd.
dc is really walkable, rock creek park is great, you can cut through the zoo . . . is really nice to go explore. you can walk through several different neighborhoods in one day. i would say avoid starbucks if you can and grab a good coffee someplace and sit down with the city paper. all museums, concerts, lectures etc are listed, so its a great source of things going on.
have fun . . .