DC has some surprising little bits of nature tucked away. The National Arboretum is a must-visit, but if you're here in July you really can't miss the Kenilworth Aquatic Gardens. Both are free admission and free parking. Unfortunately, a car is required to reach this gem.
The park preserves the wetlands that were once widespread in DC (though these are reconstructed). The showpiece part of the park is a series of small ponds with paths between them, each pond housing its own variety of water lily. They bloom around July and there is a festival the third Saturday in July each year.
Beyond the ponds there is a much larger wetlands complex with a boardwalk that takes you on a nice little jaunt. We saw a giant heron perched in a tree in the distance.
Lilies aren't the only flowers in bloom; there are gorgeous wildflowers everywhere. Wildlife is abundant as well: frogs, butterflies, and a deer that we managed to startle.
If you prefer to stay on solid ground, there is a nice little trail to the Anacostia; it is under a mile and flat (though if you're using a stroller you'd need the jogging variety with big tires). You hike along the edge of one of the wetlands, which peeks through the trees at you. At the end the river view is picturesque and there are benches to rest on.
It's recommended to get to the park early to see the maximum number of blooms. Not being morning people, we arrived in the late afternoon and it was still gorgeous. You can see the rest of the photos here.
As mentioned, you need a car to get there; be sure to look up directions in advance as it is set into a residential neighborhood.