Weekends are great times for great rates in DC. I know the chain hotels around Crystal City often have rates as low as $79/night for quality rooms. They're also just a hop skip and jump from all the monuments and museums.
Go to the zoo.
Catch a Capitals hockey game for about $10 for nosebleed seats.
Eat dinner at Luigino at 12th and H NW for superb Italian at moderate prices.
Rent a bicycle and follow the Mount Vernon Trail along the Potomac or the trail (I forget the name) that follows the canal from Georgetown up into Maryland.
Take a hike through old town Alexandria.
Best of all....all the monuments and nearly all the museums are free. Your tax dollars at work!
Oh, and I haven't been there, but the lines for the Spy museum are atrocious. I've seen them wrap around the corner. You can also try eating great tapas at Jaleo, near the spy museum and MCI center.
[This message has been edited by syzygy8 (edited 10-03-2002).]