He's not a toddler, he's almost 4.
i wouldn't take anything special with small pieces (like Lego) because you'll spend the flight trying to retrieve little pieces off the floor!
I would take some paper & colouring things (not too bothered if I can't find them on the floor). He can draw, you can tech him games.
an action figure if he has one he is attached to.
a favourite book or two.
choose a comic (with a toy) at the airport.
explain to him it's a whole day, not like just going to xyz.
is he excited about seeing the people you are visiting? Does he video call them etc??
snacks of course, but stay away from high sugar etc
you need to have a think about your attitude though, yes, it is public transport, but that doesn't mean people can just behave as they like. It is your responsibility to do your best to stop his behaviour impacting others. As well as noise, watch for him kicking the back of the seat in front or pulling on the top of it etc
I hope it's not as bad as you think it might be & you have a good holiday!!