Laurel Park is Gainesville is a huge. lovely park that, in addition to ball fields, playgrounds, and lake views, also has a water playground for kids. It only costs $1.00, and it's very well designed. The variety of sprinklers and water sprayers will keep children playing for hours. The outer edge of the fenced-in sprayground has benches and picnic tables, and the floor slopes in toward the center, so you don't have to get wet if you don't want to!
It may be a bit of a drive, but it's worth it!