By 5-6 months you have been interacting with baby lots and talking with him and playing all kinds of games. A fun twist on this is to do whisper time. Get close to your baby and whisper to him; whisper how much you love him, how beautiful you think he is, how smart he is, and how very happy you are that he is your baby. He will enjoy the feeling of closeness, the intimacy, and he will sense the beautiful things you are telling him.
This is probably best done when he is not eager for activity, perhaps at the end of another interactive activity and your baby is ready to wind down or take a break.
Of course, this activity can be done at any age with baby, but it is likely that you have not been whispering as much when he is 6 months old (and alert and active and interactive) as when he was a newborn (and new and quiet and fragile). You will see how much fun it is to change it up!