Each of the Museums has something different to offer and each takes at least a day to see just the basics.
The National Air & Space Museum has a good restaurant and several floors of rockets, planes and specialist displays that are changed quarterly.
The National Museum of History has the largest Diamond in the world, which is worth a look at...not to mention lots of hands on displys and large stuffed animals.
The National Museum of America History has an interesting feature - a large pendulum which doesn't move, the earth is moving around it - a weird sensation as you watch it knock over the candles. Plus it has displays of native americans and american culture over the years.
The National Gallery is in two buildings - East and West - they take time but are spaciuos and have a nice coffee area.
The Holocaust museum is NOT for children, it's distressing to see the pictures and films, don't go if you don't want graphic details of the horrific attrocities of war.
All the museums have great bookstores and or souvenir stores.