This park runs down the length of Rock Creek and there are miles and miles of bike trails for biking, running, or hiking. There is a horseback riding facility at the main entrance of the park.
This park saved my sanity in the big city. You drive in the entrance, and it's almost like you're transported 80 miles outside of the city. Very rarely can you even hear the traffic. There are lots of grassy areas great for games, lying on a blanket, or just enjoying the space and quiet.