Petworth Playground is a lovely spot to take a break. Enjoy a picnic, toss a ball around or just lie on the grass and shut your eyes for a while.
You'll be spoiled for sightseeing choice when in Northwest. Petworth Playground is a great place to begin here, but there are many other worthy spots to discover. White House, Arlington National Cemetery, and Lincoln Memorial are not far from the recreation and green space and will round out your day nicely.