I heard a rumor that they are going to renovate the spray park at Turtle park and make it nicer.
I can recommend the spray park in Petworth (8th and Taylor NW) but there is NO shade so you might want to bring a beach umbrella to put in the grass. The playground next to it is not as nice but there is a slide structure.
My friend at KidFriendlyDC onb Cap Hill likes to visit the new Yards Park, they have a spray fountain and a nice view of the river. Would make a nice combo outing with lunch or the play place on 8th Street SE, and Eastern Market.