I recently spent a few weeks commuting in our nation's capital, and my best route was through Rock Creek Park. I wish I had a permanent commute along a route like this!
The park was created by congress in 1890, along with Yosemite National Park. The park is a long valley that pretty much divides the district in half. Most access points are very steep, and I see many more riders walking those hills than riding them. (At the end of my three weeks in the District, I'm proud to say I was regularly climbing those hills.)
Beach Drive, the roadway at the center of the park, has no shoulders to speak of. While roadies who can maintain 25mph do take those lanes, I don't recommend doing that. For those who want a great road surface free of cars, Beach Drive is closed to motor traffic on the weekends north of the Blagden Avenue access point.
The path that runs through the park is great for casual riding, or short rides, but not so great for longer workouts. The surface is paved, but it's very uneven. My Kojak treadless tires weren't happy, transmitting every crack and dimple to my hands. (I kept them a little low, around 90psi instead of the rated 110.) I quickly learned to anticipate the worst potholes on my five-mile ride.
Near the national zoo, the trail narrows to a sidewalk that's maybe two feet wide. I saw a few folks taking the lane in the tunnel or riding that narrow sidewalk; but if you're here before 6pm, the zoo grounds are unlocked and there's a trail that leads you up the hill and around to the other side of the tunnel, and that's a much more pleasant ride. (That seasonally change to 8pm, I think in the late spring and summer.)
Those hills I mentioned at the access points? They're in the park too. Lots of short, quick rolling hills on the trail. In some places, the trail overlooks the roadway with no railing, and I can see that being a little dizzying to anyone with a problem with heights. The three-speed District bikeshare bikes don't cut it on this trail at all. My Bike Friday, equipped with a climbing-specific drivetrain, performed excellently on these short hills.
I've ridden the Rock Creek trail in the past, but never this regularly. Overall, I had a blast, and my folding bike will always be in my trunk when I travel to the capitol.