Letting the water out of the fire hydrant on your block on a hot summer day as a means of cooling off. This is often done in poor urban "ghetto" areas because nobody in the projects wants to spend 20 bucks to go to great america or six flags or wherever.
Fred: It's really hot out, you wanna go to raging waters?
Tom: Naw, I don't have any money and I've got a better idea.
*Opens fire hydrant*
Fred: Woot! A ghetto water park!