R.G.
Well, there's not much you can do with a 3 1/2 month old baby...If you have a swing, or bouncy seat with toys hanging in front of the baby, or even laying him/her on their back in a playpen with some toys lying around. I don't know what they're called, but the mats that have hanging toys that come up on top, crib gym I think, those are great. You can also try putting the baby on their tummy too - so they can build their neck strength. But at 3 and 1/2 months old, they need to be held and played with a lot. As they get older, that changes...Hope this helps!