There isn't enough time to see everything along the National Mall. It is deceiving... it looks easy to walk between monuments but the distances can be long. Wear comfortable shoes... bring water and snacks because food and drinks at the refreshment stands are very expensive. Parking is very expensive... we paid $22 for a couple of hours (same price for 1 hour or all day). There is a great website to scout out parking ahead of time: http://washingtondc.bestparking.com/. We had a large group so paying for parking was still cheaper than all taking the Metro. All of the museums are free. The free maps of the museums aren't real specific so it would be wise to research what you want to see at each museum ahead of time to make the most of your time at each place.
There is construction going on right now which pretty much ruins any pictures you want to take. Still, there are few places that have so many free attractions in one area!