Everyone who comes to Washington wants to visit the Smithsonian. That means different things for different people. Some think it's the museum where you see the lunar rover. Some where you see the Hope Diamond or First Lady dresses. Some are looking for a Renoir. In fact, the Smithsonian Institution is a network of 19 museums. These include famous museums such as Air and Space, and Natural History (but not the National Gallery of Art) and lesser-known museums such as the Postal Museum.
Never heard of the Postal Museum? It's actually pretty new and quite small, but I've gotten to know it pretty well the last few months for one reason: the semi-truck. The museum is home to a life size semi truck cab (semis travel the "star route" hauling mail cross-country). Joseph LOVES this truck. We once sat in it for over an hour. We try to hit the museum when it's not too crowded (which isn't too hard at this little-known attraction) so that there aren't too many other competing kids.
The best part, of course, is that it's free just like the rest of the Smithsonian museums. I'm really going to miss that about Washington. That makes it okay to go to the Postal Museum twice a week, or to leave Natural History after ten minutes because the dinosaurs are too scary.