If you're the type of person who can go non-stop all day you could pack a lot in. The Smithsonian's are open to 5 or 5:30pm and most of the memorials are open till midnight, so if you really want to pack it in do the museums during the day and see the memorials at night.

IMHO the order I'd rank places to visit:
Museums:
American History (Transportation exibit on the lower floor)
Natural History (Hope Diamond)
Air and Space Museum
Holocaust museum
Spy Museum

Memorials:
Washington (get a free ticket on-line at NPS.gov if you want to go up)
Lincoln
Jefferson
WWII
FDR (as stated before, very cool at night)

Other items of note:
As mentioned use Metro, DC is full of one-way streets and little parking
The National Archives (If you go see the new Nicholas Cage movie "National Treasure" you'll feel in-the-know!)
Arlington Cemetery - Metro has a stop
Old Europe for dinner up in Georgetown - Idine miles too!
The national arboretum near the capitol if that's something that would interest you
The West building of the national gallery of art (if you really just want the good stuff, go to the second floor to the European Masters section. Skip the entire East building unless you really like modern art )
If you really want to tour of the Capitol (I'd pass if you only have 2.5 days) call your Congressman to arrange a personal tour by one of his/her aids.

When you need a break from all the walking you'll be doing, find a bench on the mall and just sit and watch. Everyone from Senators, college students, foreign tourists and homeless people can be seen.