ISO Recs for amusing two young girls in Washington

Bill Coe

We will soon receive a visit by family from California. They are bringing two girls, sisters 10 years old and 8 -- real spitfires, vigorous, extremely active and athletic but with a love of reading and learning. Naturally, we want to entertain them in the city. Perhaps my neighbors can suggest fun, constructive things for them to do here. If so, please send me your top two ideas.  
We already know the usual options for museums and other "static" attractions. I just have no idea what might engage these girls and make their visit a memorable one. Thanks in advance for any useful recommendations! 
Bill Coe
Van Ness  
bceedeec @ gmail dot com