I vote no for PlayDough. It is sometimes rejected by security, but beyond that it has a bad smell, and it stains. I know someone who ended up with several colourful stains on her suit after a flight, (she didn't realise that someone had left tiny balls behind on the seat) and they could not be removed.

I have no experience with Crayola Modelling Clay, but apparently it is odourless and does not stain, so I would strongly suggest that instead.

I also suggest felt pictures, puzzle blocks, Rubix cube (even for those too young to solve one), pipe cleaners, sticky tape, post it notes, and anything which doesn't make a noise and which doesn't have small parts.