Daingerfield Island is a small peninsula one mile south of Reagan National Airport. It has a sailing marina, the "Indigo Landing restaurant and a swampy nature area right next to Mount Vernon Trail:
The little island is as well a place for plane maniacs. It's not as popular as Gravelly Point, however, planes approaching the airport from the south can easily be spotted from here.
More information about Dangerfield Island is on rundc.com.