Best time to go: 04/01 - 09/30
Just half an hour north of DC are two public lakeside retreats with playgrounds, boat rentals, picnic areas, and an abundance of history and natural attractions.
Seneca Creek is the closest park to the city (less than half an hour from downtown) and has shady picnic spots and a lovely lake with a perimeter trail. There is no boat rental at Seneca Creek Park, but there are plenty of spots to fish, and you can put in for free at the public boat ramp.
The nearby Seneca homestead includes an historic log cabin.
Black hill park offers more complete amenities, with a nature center and boat rental facility (including canoes, kayaks, rowboats and paddle boats). All may be had for $15-$30 per hour.
Both parks feature ample playgrounds.