Stacking cups are awesome. Even now, at 21 months, J enjoys them.
J first learned to tear down a tower that mom/dad created. At first he was a very careful little boy, removing one cup at a time (starting around 8 or 9 months). I also used the cups to help teach object permanence (the idea that an object doesn’t disappear simply because we can’t see it). I would place a small toy under one of the cups and have him find it again. Then I added two cups, hinding the small toy underneath one of them so that he had to find which cup help the surprise. After working up to 3 and 4 cups and he was quick to find the surprise underneath, I started hiding the surprise while stacking,… basically any way to make the game new and interesting.
Of course eventually he learned to stack the cups himself, nest the cups (I use both activities to emphasize size – big/small), fill cups with the matching color pompoms, fill and spill in the bathtub,… There are so many possibilities, long lasting possibilities!