What are some fun things to do in Washington D.C.?

I am going to Washington DC in 3 days I don't really know what to do there besides see landmarks. I'm only 11 so I want to do stuff besides see the white house and stuff but I don't really know what to do there! It will ne me my 5 year old sister, 7 year old brother, and me and my twin sister. We are going with our grandparents and they said we could choose what we could do. Thanks Please help!