At the Waterfront Park in Bladensburg, one can board a pontoon boat and see the wild, unspoiled part of the Anacostia River. Here are blue herons, geese, turtles, and other wildlife. It looks like it could be in the Deep South. The boat makes its way downstream, passes beneath the Highway 50 bridge (New York Ave), turns about, and heads back. It takes about an hour.
You can fish, rent a kayak, launch your own boat, and even take in a local concert. The boat rides are on from April to October, and go every day except Monday. There are several per day, and they are free. Check the website below for exact times. And arrive early, because the spots can fill up very soon.
It's hard to believe that such a place exists inside the Beltway. If you're in Washington or Baltimore, and just want a change of scene, then check this out.