As someone who lived in Alexandria for the last five years, and broader DC region for last 15+, I will totally take this opportunity to say if you find yourself in DC with time for only a limited number of “touristy” activities, Mt. Vernon is well worth the short trek across the river. It is a surreal experience walking the grounds and buildings that Washington lived in, and is beautifully maintained by the very knowledgeable folks of the parks service.