I work in DC, really close to L'enfant Plaza and the Capitol Holiday Inn. Both are very nice establishments (I've stayed in them both). The advantage to those places is that you won't need to get on Metro and can walk to most of the Smithsonian museums. I'm not sure how much they cost though...
I would really recommend the Residence Inn in Arlington (Rosslyn). Like a PP said, it has breakfast (eggs, a waffle maker, juices, etc.) each morning. They also have yummy cookies at the front desk all day & night. I stayed there on a weekend & IIRC the rate was only about $90 a night or so.
DC's a great vacation spot, but since I work there, I get envious seeing families on vacation when I'm working

Plus people that work here just don't take advantage of all the fabulous things in DC. Enjoy!