Peekaboo, make faces in the mirror, read to them, put jingle toys on their wrists and feet, get a tummy time pad that fills with water, just take them outside and let them look around. There’s not to much you can do at this age.
Dance around and sing with them, tell them the latest gossip- but really, I read having conversations with them and responding to the noises they might make back is really great for language and brain development
Sensory toys or flash cards, playmat , rhymes , reading books