Puzzles!
Theo has one puzzle in particular that he does a lot and knows well and he has two that challenge him but he is willing to try more than once. To keep his interest I frequently switch out puzzles.
This way his brain is always being challenged. Now I'm not saying a 50 piece puzzle BUT I am saying challenge to an extent that's good for him (all children will be different). I never have them all out at the same time, this could be too overwhelming for him because he is so young.
It’s good for young children to see things more than once-this gives them time to process and learn through repetition. As “simple” as puzzles may seem for an activity, it will sure give their little noggins a workout!