The Lincoln Memorial. The steps offer the best view of the mall The Washington Monument is very cool.
The Korean war memorial is tool. It was supposed to have 38 soldiers to correspond with the 38th parallel but they ran out of money. The Marine Memorial, the other side of the Key bridge from Washington is amazing too. There is lots of room in the park for kids to run and play. Frequently you see people playing Frisbee and the like there. The list is long. I walked the entire mall the last time I was there, but that is 8 miles if you go all the way around. The White house is great, but you need to ask your Congressman for tickets months in advance.. For museums I agree with Cassie. Air and Space is awesome.