We were there for 6 days from 29th - April 4th and had a fantastic time there - the only problem was the weather - it was more winter like than spring , with temperatures hardly exceeding 10 degrees Celsius (though at times it felt much colder than that, coming from Dubai where it is currently 30 degrees C and climbing !!!)
Another aspect was the ENORMOUS crowds at all the tourist places - the Lincoln Memorial was simply swarmed with a 10 person thick wall of humanity and it was a real struggle to get close to the statue !!! Mostly school groups, i guess. Similar situation at the Archives where the wait to get in was an hour and a half, the Space Museum, Natural History , Holocause Museum etc, which were simply swamped wtih people !! The lines to get did take away some of the sheen from the trip, since we never expected such crowds. However, once in, the crowds dissipated except for few spots like the Moon Rock at the Space Museum, Rotunda at the Archives, Hope Diamond exhibit at Natural History etc, which was always surrounded !! We went a second time to Air and Space , this time at 5.30 and it was MUCH better !!! Most enjoyable was the visit to the House Galleries at the Capitol and the Courtroom visit at Supreme Court - gave a very good idea of how American Judiciary and Legislature functions!!!
Other places visited include Capitol (FABULOUS !!!!!!!!!), Supreme Court (INCREDIBLE ARCHITECTURE !!), Old Post Office, Old Town Alexandria, Arlington Cemetry, Pentagon Memorial, Udvar Hazy Museum (far less crowded and much better), Georgetown, Kennedy Centre. Walked from Washington Circle to the Capitol through Penn Avenue - great experience !!! Also did the highly enjoyable Embassy Walk through Mass. Avenue - very interesting!!
Would really thank warthog, arlingtonman, darrel and other experts for all their advice and input which made this trip such a memorable one for us !!!!