The Kenilworth Aquatic Gardens are a lovely (and free) escape from the hustle and bustle of the city if you enjoy nature and wildlife. Although they have different plants in bloom year round, I visited last year during the annual Lotus and Water Lily Festival in July. The weather lotus flowers were so beautiful and definitely worth a trip.
In addition to the flowers, there are lots of different paths to explore, and plenty of shaded areas if you decide to have a picnic. There is also a visitor center with a brief overview of the history of the gardens and the peak seasons for various flowers. Between the garden stroll, museum, and possibly a picnic, I would plan to spend 1-2 hours here.
If you are a runner/biker, the gardens connect to the Anacostia Riverwalk trail (see my post on this trail to learn more). If you have a car, there is also plenty of parking.