The best place I can suggest is Macka park between Nisantasi and Taksim. It has two levels joined by stairs and a teleferek, which I recommend highly. It's one of the weirder types of public transportation in the city. On the lower level, to the left of the big fountain you'll find a lot of soccer games and the like during the summer. The other famous parks are all a bit too fancy for soccer games.
Another good bet is the courtyard in front of the Besiktas ferry. The games that go down here are a bit more serious and attract an older crowd than Macka park, but again, the location is great and there's always a game going on. Watch out for skateboarders, who also use the area to practice their grinds.
