Didn't say when you are going (dates)...
but, if Congress will still be in session, call/write your Congressman and Senator to get passes into the Senate/House gallery. You MUST plan ahead for this! Also, get them (Congressman/Senate) to get the SPECIAL WHITE HOUSE PASS to tour the White House (if you wait in line for the general public pass, you will waste a LOT of time...get the priviledged pass and don't wait in line!). These passes are very limited and can only be obtained from your represenative, so start asking NOW!
Do NOT get a rent car in DC! The traffic is worst than Houston and there are NO PARKING SPACES in the summer. Make sure you ride the METRO subway..its clean and safe! Neat way to get around town.
I used to go to DC about 4 times a year on business for over 20 years. All the places others suggested are right on! Smithsonian is great. THe National Zoo is over-rated, IMHO. Very good seafood restaurants along Water Street.
Caution: Do be very careful after dark on the streets of DC. It has a very high crime rate with lots of muggings. Being in crowds is OK but avoid dark streets.