Simple kinetic toys, of the sort where you put a ball in a hole and it pops out somewhere else. Same pleasure as a marble run, without the choking hazard.

Adult mimicry toys, like play kitchen, doctors bag, etc. With the right setup and creative inspiration they can learn to putter about inventing simple tasks and executing them.

https://www.amazon.com/HABA-Musical-Eggs-Acoustic-Germany/dp...

He's also enjoying the Mega Bloks someone got him though he's mostly just sticking them together and throwing them.

Or take them to a library that has toys, see what they play with, and then go buy that.