Pick someone your child knows and ask what would make that person happy. Steer towards small and doable: a drawing, tidying their shoes, a note.
Ask why they think that person would like it. That question is the activity — it requires imagining a preference different from their own.
Carry the plan out together and watch the reaction.
Choosing a gift for someone else requires modelling what they want rather than what you want, which is perspective-taking in its most concrete form. Seeing the reaction closes the loop and makes the abstract idea of another mind briefly visible.