Little Guy picked up a bunch of red drinking cups about two weeks and started playing with them. He was so obsessed with it that I ordered him a set of "official" stacking cups.
If you think this is easy you should grab some cups and try it. Also, there is an entire set of rules associated with this sport. Yes, this is a sport. It is incredible for hand-eye coordination. Little Guy's concentration skills have definitely improved. I have been amazed at how much time he has spent practicing at this. Sometimes he will practice an hour or two at a time. Great discipline!