We stayed at a Holiday Inn Express in Fairfax, Virginia for $68/night. We were about 10 min from a Metro station (free parking on weekends.) We did the unlimited pass for each day, which was I think $32/each. You can try to figure out a basic plan for your trip and see if it is cheaper to pay per ride, but for us it was cheapest to buy the all-day pass. Food is very expensive (IMO more than CS at WDW) in Washington, and we took full advantage of the free breakfast in our hotel - they had waffles, omlets, doughnuts, bagels and muffins, cereal, fruit... very filling breakfast for free.
Couple things I would recommend:
1) Purchase tickets to go up the Washington Monument online before your trip. I did not do this thinking we would get the free tickets while there, but the free tickets were gone almost immediately both days we were there, and we didn't get to go up.
2) Contact your local House Rep and schedule some tours. These are free. We did a tour of the Capitol building that was incredible. Next time we go, we are going to try for the Rotunda tour there as well.
My kids really enjoyed the history and air and space museums. The natural history was good too, but not as good as our local museum. The zoo was also not nearly as good as our local one, and the smell of cat spray knocks you out when going in the front gate.

But the pandas are cool, and made it worth the trip out there.