My mother is a developmental psychologist specialised in babies / toddlers. According to her most research showed that behavior of the parents was a far far bigger factor in the speed of development. And on the toys side of things that it's mostly about allowing for creativity in play, meaning kids want to invent their own game with whatever you give them.
Simple wooden building blocks or a stack of cups that fit in each other already allow for that for early age, no need for more advanced toys / battery driven things.
Simple wooden building blocks or a stack of cups that fit in each other already allow for that for early age, no need for more advanced toys / battery driven things.