If your one of the expatiate who likes to play different sports then the ground on Kalamegdon Fortress if just for you. You could play anything from Cricket,Ultimate Frisbee,Rugby,Archery, some 10 odd more sports and even practice tai chi ;) . These are group of enthusiastic Serbians/Expatriate who come to practice these sports.
They play at the ex beer fest venue just behind the zoo from 5 pm almost every evening. Since I play cricket its a good place to meet like minded people and enjoy a few drink post the trainings.