This Sunday, while my wife was travelling, I took the kids to the Chocolate Festival in Old Town Fairfax. I didn't even know there wad an Old Town Fairfax, much less a chocolate festival. It was okay overall, but the best was seeing any type of chocolate you could imagine, and letting the kids pick something for themselves and their teacher.
For finding cheap or free stuff going on nearby with the kids, I have to say About.com has consistently been the best. I tried si.edu (the Smithsonian Website) which is also good, but a little hard to navigate, partly because they have so much going on. At About I did a search of what to do with my kids this weekend, and a bunch of items came up. dc.about.com is really the best place to go.
I'll write about some specific things you must do if you're in DC. These are things I have shared with kids, teens and adults for years, and everyone is always impressed.