National Archives in downtown DC. The free exhibit runs through January 3 and is all about the government's effect on the American diet. It sounds like a rather dry subject, I admit, but don't let that prevent you from going. I found a lot of the displays were quite fascinating and much ofthe subjects were centered on gardening/growing of food in this country and how the government has had a hand in guiding it.
For instance, did you know the US government gave out free seeds from 1839-1924 just for the asking? A log book is on display at the exhibit as well as some of the seed packs. Not just crops like wheat, just also cosmos flowers and more ornamental plants. The practice ended after the American Seed Trade Association successfully lobbied against the practice as clearly this hurt the seed trade.
Another fact I had not known was that while the folks back home were on rations during World War II and told to grow their own food, our boys overseas were alloted 5,000 calories per soldier and the average joe came back 15-20 pounds heavier!
If you go, be sure to stop by the gift shop on your way out. They have some nifty cookbook reprints and retro-style items. I have my eye on a tin mug stamped "Dig for Victory" that will make a great holiday gift for a couple folks on my list.
BTW absolutely NO photography is allowed inside the exhibit, so leave the camera at home.