The past few weeks I've been over to Meridian Hill Park (aka Malcolm X Park) on Sundays, and it's a pretty good time.
The main event is a drum circle and people dancing, but there's a lot for non-drum enthusiasts as well: every time there's also lots of families out, people picnicking on the grass, kids playing ball, folks walking on ropes and more. It's very diverse and everybody has a good time. Two weekends ago they had a clothing swap, which was pretty cool.
It only happens during warm months, so it might be ending soon! Go take a look if you haven't been already. It happens every Sunday evening from about 3 until 9, weather permitting. Here's a ton of photos to give you an idea.
Photo by stevendamron, used under a Creative Commons license