Not sure at all, but one thing that strikes me is when you’re purposely “thinking really hard,” you’re probably in a state akin to vigilance. You’re trying to assess ideas quickly and move to the next one. If you’re playing with legos, an idea can just sit there in the back of your mind and tumble about a little bit into different configurations and orientations.

I think another commonality is physical activity: walking, playing, washing. There may be some chemical thing going on with muscle activity but also subjectively I have a suspicion that these sorts of activities are essentially pumping noise into your cognitive processes, helping divergent thinking, while also keeping your attention sufficiently occupied to achieve the “non-vigilant” posture toward those ideas.