SAHMs - what do you DO with your babies?

I am a new SAHM. My 3 year old is still in care full time (though we are scaling it back soon), so right now it’s just me and my 4.5 month old at home during the day. I don’t know what to do with him! I feel like he’s bored, but he’s hard to take out of the house because a) he screams non stop on the car and b) the weather is terrible. On nice days we get out for walks, but other than that our days are quiet and I think we both would like some more activity.