One of the hidden gems in the DC area is Theodore Roosevelt Island (I would link to the official National Park Service page, but due to the Federal government shutdown, the website has been taken down). In September we made a weekend visit to the tromp around the island, watch birds & see some nature. A few photos are below. Unfortunately the island park is closed to the public for now, but it is worth a visit when parks are opened again.
Photo by Patrick Jones - Theodore Roosevelt Island