DC's best kept
Kenilworth Park and Aquatic Gardens2
Why locals love it
While the National Arboretum is visited by many, the more rustic marshes and ponds just across the Anacostia River are a lesser visited treasure of the National Park Service. In the mid 1920's, Congress made efforts to preserve forests and waterways (DC was once a swamp, after all). These wetlands have stayed an unkempt majesty for the outdoorsy.
Why you should visit it
It is rare to encounter nature just within a city in this more subtle, unmanicured fashion. Though there is a small boardwalk for visitors to encounter the marshes, most of the area is ripe with trees, plants, flowers, animals, and insects. Whether you're a birdwatcher, hiker, explorer, or the like, no visit to the Aquatic Gardens is the same.
Keep your eyes peeled for turtles in the ponds, lotus and water lilies, egrets, blue cranes, herons, and the like.