If American history class wasn't really your thing (read: you slept through it), here's a chance to get caught up. Washington, D.C. will be hosting more than 80 free walking and bike tours this Saturday and Sunday.
Cultural Tourism DC, a nonprofit coalition of cultural and neighborhood organizations, is sponsoring the event. The breadth and wealth of subjects is almost overwhelming (the tours span 18 neighborhoods), but here are a few that stood out: "Art on Line: The Story Behind the Artwork at Metro Stations," "Can You Dig It? The Past, Present, and Future of Mt. Zion Cemetery," and "The National Mall: What the Memorials Don't Tell You," where you can be a guinea pig for a future self-guided tour (plus, it sounds like there might be some National Treasure action.)
Meeting points, times, and guides vary - some tours require registration (although all the tours are free). You can get all the info at culturaltourismdc.org.