Because people are so afraid of getting alzheimer's disease, there has been a lot of interest in puzzles and other games that are supposed to keep your brain sharp. Some people do sudoko, some do set puzzles, and I'm sure there are dozens of other such activities.
Those are great, but all you really need is a boy who recently turned 4-years-old, and who prefers toys that are made for children over 6 years old.
Currently, Jack is obsessed with Transformers. As you probably gathered from the name, they transform from some kind of vehicle into a robot... and back again.... like a puzzle.
This one is pretty easy, and he can do most of the transforming himself. I only have to help with the final step in transforming from truck to robot.
This one is a little bit harder, so Jack needs a little more help with it.... though he doesn't play with it that much because he informed me that it's a Decepticon ("bad guy"). This one is a bit more complicated. Jack tries, but he gets frustrated and pops the pieces off. So, I have to put the pieces back on and then transform it.... from robot to jet.... and back and forth again. (He likes this one, even though it is also a Decepticon, because "it's a cool jet.")
This one is his favorite though, and it's really tricky. (OK, it's not as bad as a Rubix cube, but it's pretty complicated.) Jack doesn't even try. He just hands it to me and says, "Mom, can you transform this?".... about 20 times a day. I'm getting pretty good at it, but it still keeps me (and my brain) on my toes.