In one scene in my YA fantasy my characters are at a party on a beach. One piece of the setting is a group of teenagers playing a game, which my main characters later join. The game itself is not the focus of the scene, and if this were in a modern story I would label it as volleyball or frisbee and leave it as that. Since this is fantasy in a world that is not ours, I do not have that option.
My question is should I briefly describe a game that is common to readers, or should I take the time to develop a new one? My concern is either boring the reader or taking away from the flow of the story.