Rock Creek Park is a National Park located in Washington, D.C. It is the largest urban park in the United States, and also one of the few parks established before the founding of the National Park Service.
Just above where the Potomac meets the Anacostia (i.e., current DC's downtown), Rock Creek empties into the Potomac. The imaginatively named "Rock Creek" is exactly that: little more than a meandering creek that is, well, somewhat rocky. It meanders through the valley of a pleasantly wooded area, with deer and other cute fauna; florae consisted mainly of deciduous trees and their attendant underbrush.
In 1703, a trading post was established at the mouth of the creek; soon the trading ports of Georgetown and Alexandria (in Maryland and Virginia, respectively) were thriving ports on the Potomac River. This in turn aided the settlement of farmers in the area. Naturally, mills were needed to grind the grain (and other things that needed grinding, like bone!), and Rock Creek was the logical place to build them. Of the nineteen mills that were built, only one survives today, the historic Pierce Mill (owned, surprisingly enough, by Isaac Pierce; his carriage house now serves as the Rock Creek Gallery).
Somewhere along the line, the United States was founded. This was considered a big deal at the time. For various reasons, they eventually decided to make a new city for their center of government; this was, of course, Washington, D.C.. Thus, the area that would become Rock Creek Park became the property of the Federal Government, rather than Maryland. The trees were unavailable for comment at the time, although a sparrow was reported to have said "cheep".
Around the 1880's, the Industrial Revolution caused water wheels to become obsolete. The mills along the creek gradually fell into disrepair and nothing came along to replace them.
In 1890, Rock Creek Park was established. It set aside 1,754 acres of land "for the benefit and enjoyment of the people of the United States", according to an act passed by Congress. This makes it the oldest urban park in the National Park Service, and it served as a direct inspiration for the establishment of the aforementioned NPS, into which it was incorporated after its creation. It's also more than twice as large as Central Park (although this comparison isn't entirely fair. Rock Creek is much less accessible; while Central Park was designed with humans in mind, Rock Creek Park was not so much "designed" as "set aside", thus there aren't that many paths, park benches, etc. -- it's more like a forest, and less like a park).
Should you feel that wandering through the woods is not a pleasant enough activity, there are some historical buildings you can visit inside the park. Pierce Mill is the most cited landmark in the park. If you'd like to look at an authentic 19th century mill, this is the place for you! Other than the exciting mill action, there isn't much else in the park. There's a nature center, with a planetarium; but, let's face it: if you're in DC, you should go to the Smithsonian Institution for all your educational needs. Unless you feel a burning desire to learn more about the park -- but really, folks, it's just a bunch of trees. Oh, and, according to the National Park Service, "Rock Creek Park supposedly has the largest density of raccoons in the US". Fun times. There're also foxes and deer, but no bears.
The best thing to do in the park, however, is travel through it. Many of the roads that go through Rock Creek Park are closed on weekends for bikers, joggers, rollerbladers, and anything that isn't motorized. The parts that don't close have a nice paved trail running along side them. It's nice to be in a thickly wooded area in the middle of a city; if there aren't any cars for a while, you might forget that you're even in a metropolitan area at all.
And, speaking of cars, Rock Creek Park also contains a fairly major road. Called the "Rock Creek Parkway" (creativity in naming strikes again!), it's two lanes in each direction, roughly following the creek itself. During rush hour, it become four lanes in one direction, helping to funnel traffic to and from downtown. It's a fun, twisting drive, which scares the heck out of people who haven't been on it before -- the exits aren't clearly marked, cars often go 50 mph along its crooked roads, and the two/four lane thing gets really confusing.
In recent years, there have been several battles over development in the park. The most prominent one has been over cell phone towers. Proponents point out that cell phones could be life-saving in an emergency; opponents point to the unspoiled beauty of the land. Cynics might be observed to grumble about the astonishing coincidence that many of the congressmen who made the proposal also happen to commute daily through the park. For now, there are no towers, but time only knows...
Also, Rock Creek Park is a great place to hide a body (see: Chandra Levy, whose remains (at least, police think they're hers) were found in the park months after her disappearance).
But, aside from traffic, politicians, and murder, Rock Creek Park is still a great place. If you're ever in DC, go to the park, walk among the trees, and try to forget about the idiots who are controlling the US less than two or three miles away.
The NPS web page: http://www.nps.gov/rocr/