when my son was 4-5 months old he loved playing peek a boo, Air plane laying on my feet, me on my back on the couch, bed or floor, being hed in a rocking chair him standing on my lap and my holding him at his armpits or his hand/arms and while rocking like that I made animal noises, car noises, beeping. He liked to lay on his stomach on the floor with a child safety mirror and hear tape recordings of himself babbling and laughing, He loved the tv show "rugrats" (the one with Chuckie and Tommy as the babies), a friends daughter at this age loved teletubies.