My one single piece of advice for a visit to Washington Monument is to get in line early! They start handing out tickets to the observation deck at 8:30. We arrived at 7:15 and there was already a line. We later learned we could have ordered these on line. If you do not order tickets on line, then plan to arrive early as tickets go fast.
The elevator will take you up to the observation deck. The views are amazing. I was able to see the White House and other monuments around the city. I also saw an amazing view of the river. There are stairs that you can walk down but for some reason they were closed off the day I went.
My biggest complaint is some of the windows could have really used a thorough cleaning, inside and out. I do not know how often they are clean but I saw a lot of finger prints on the windows.
I enjoyed the views though and it was worth the extra effort to line up early and get a ticket. When I was there lining up early there were a number of homeless people walking around. It was OK though, they did not bother you.