4 months old activities

Ive run out of things to do during my 4 month old’s wake period. He used to hate tummy time but now that he has learnt to flip, he be flipping most of the time and always on tummy time. Other than that, we put him on a play gym with hanging toys, rocker with hanging toys, talk to him, sing to him. Aside from reading books, what else should i do to fill up his wake period ? His routine is usually milk - playtime - nap