Fort Dupont was indeed a fort -- part of a circle of outposts on high points around the edge of DC during the Civil War. You can still see some of the earthworks on the eastern side of the park, near Alabama Avenue SE (see our map). From some high points in the park (like the hill behind the ice rink), you can see the Capital -- though the soldiers were likely facing in the other direction when they were guarding the city.
Although the area was farmland around the time of the Civil War, a large portion of Fort Dupont is now wooded. And Pope Branch flows through the park on its way to the Anacostia River.
An 8 mile trail called the Fort Circle Park National Recreation Trail links Fort Dupont to several other small fort-based parks east of the Anacostia, running from Fort Mahan just north of Benning Road NE to Fort Stanton, south of Good Hope Road SE. Parts of the trail are paved, but much is a natural-surface trail. And, it's the only natural-surface trail within the DC city limits that allows mountain bikes.
Hiking: There aren't a lot of good options for loops in the park -- most of the trails are out-and-back unless you circle back along the road. A trail leaving from the parking lot near the main entrance goes down to follow a small stream. Others climb through the hills. Go exploring and see what you find!
Bikes: Are allowed on all trails in the park. If you're biking the Fort Circle Park Trail, check out this advice, or the page sponsored by the Mid-Atlantic Offroad Enthusiasts -- there are a few tricky intersections.
Dogs: Are allowed, on leash. Please scoop your poop.
Other activities: The sports complex along Ely Place includes tennis and basketball courts and athletic fields; an indoor ice rink offers skating all winter. There's also a community garden.
Getting there: The closest metro station to Fort Dupont Park is Benning Road. You can walk the .4 mile to Fort Chaplin park on East Capitol St. and make your way down the Fort Circle Park Trail (about a mile total to the edge of Fort Dupont Park, but much of it is wooded).
For a ride right to the main entrance to Fort Dupont, take the V7/V8 bus from the Minnesota Avenue station to Randle Circle (Minnesota Ave. and Massachusetts Ave.).