If you are visiting DC then Arlington Cemetery is a must to see. We were there in December and even though we are from Australia we were so touched at the care and remembrance shown by having wreaths laid on every head stone. It was very moving and interesting reading the inscriptions on the old stones.
My family and I visited Washington DC in December while on holiday from Australia and I was very surprised how big the building and the statue were. We spent about 3 hours having a look around The Lincoln Memorial and looking at the other memorials along the reflecting pool. I would suggest that you spend some time there in the day time and then take another walk looking at the memorials in the dark as the lights make each of them take on a different feeling.