One of the nice things about having younger kids over to our house regularly is that Alleluia can be the "big girl," but it also means that she can play with toys we haven't had out in a while.
When we recently got this big gumball toy out of storage I was surprised to see her so engaged. As a baby she liked to watch the balls spiral down the globe, usually with my help. But as a toddler now, using this toy by herself, she approached it as a problem-to-be-solved.
The balls didn't come out easily. Instead of getting frustrated, she shifted into problem-solver mode. (They didn't come out easily because most of the time about three balls were crowding the little port at the bottom, making it hard to remove one).
She fiddled with some of the levers but learned that the best way to get all of the balls out was just to dump them out, holding the machine upside down.
Once she solved that problem (and filled the machine up and dumped the balls out again and again), she wanted to sort the balls into groups (those that rattled and those that didn't) and hold them up to the sunlight to see which ones were "see through."
Try getting some older toys and works out of storage and see what your children do. It's fun to observe their novel approaches to a familiar toy. Compare this to how they were working with the toy just 6 months ago--or even longer. Their growth and development becomes apparent.