Hold your baby about twenty-five centimetres from your face — roughly the distance at which newborn vision is sharpest — when they are calm and alert.
Look at them and let them look back. Smile, raise your eyebrows, and then wait.
When they move their mouth, widen their eyes or make a sound, respond to it as if it were a comment. That exchange is the activity; nothing else is needed.
This back-and-forth is what researchers call serve and return, and it is the single most studied ingredient of early brain development. Responding to a baby's signal wires the expectation that communication works, which shapes everything built on top of it.