The National Zoo!!!!!! The pandas were adorable.
Museum of Natural History. It had/has the Hope Diamond
The Air and Space Museum
We were there for the National Spelling Bee around Memorial Day weekend one year, and it was packed. Be advised that many of the most interesting things to see will be very difficult to get into and very crowded. Some things require tickets that can only be got by standing in line very early in the morning in the hopes of getting a ticket, (that may or may not be avaliable), or the tickets have to be ordered ahead of time.
The Library of Congress was not worth the time. They had tours available but were not that interesting because all the most interesting parts were off limits to the general public.
Be sure and take snacks and bottled water while walking around. There was a limited number of food stands although there was water available almost anywhere, but you can imagine the prices.
There is a great food court at Union Station in the Metro line, and China town is just down the street (a short walk) from the Grand Hyatt, which has a Metro stop coming up into it.
There is also a trolly that goes around DC and stops at different places like Arlingtion. It's pretty reasonable and you buy a ticket and get on and off at the different stops at the sights you want to see.
Union Station is an attraction in itself, IMHO. Beautiful old building, shops and so forth. Walking distance from the Capitol building.