DC is one of my all time favorite cities and you can get alot done in 48 hours. So many Smithsonian museums to visit on the National Mall during the day (plus various monuments) and then dining in town. I think you'd have to really plan what you want to see.

I'll add that I think the Capitol building is open (again) for tours. I've done it and it was really interesting but you do get pushed through pretty quickly. You must plan ahead for tickets.