Anything that behaves like an object, would be the obvious answer. GUI elements, things in games or simulations. More abstractly, an object's a namespace with p

Anything that behaves like an object, would be the obvious answer. GUI elements, things in games or simulations. More abstractly, an object's a namespace with persistent properties, so for something that has a persistent, possibly mutable, state, an object is the metaphor you want. Whether data structures should be objects rather than primitives depends very much on the data.