Arlington is served by the Orange, Yellow, and Blue lines. The front and rear of each train tell you which color line that train is following. A message on the train's side tells the direction in which it is travelling. The Metro bus system is also easy and inexpensive. Check with the front desk of your hotel for schedules and bus numbers.
Museums become very crowded from just before lunch through the afternoon.
Visit monuments and memorials in the evening. They are very attractive when they are lit up and they are less crowded.