Gently blow on your baby's face and hair. Ask them, "Do you feel the wind?" Notice if they like the sensation. If so, blow with long and short breaths. Play as long as baby is interested and enjoying the time with you.
Is your baby starting to babble? Use bathtime as an opportunity to babble about "b" if they say, "ba" you can say, "Baby takes a bath in bubbles! Ba-ba! If they respond back with more babbles, copy them right back!