So why does taking a shower help us think? Or driving a car on the highway to work? It has to be more then just getting blood moving?
In my opinion, it's not necessarily about getting blood moving. It's rather about relaxed (a.k.a. diffused[0]) thinking. Let it be driving a car on the highway, taking a shower or spending time in green spaces - both make our thoughts roam freely, thus, more creatively.
For me, the more important question is: why & how our thoughts start to wander around when we are doing such activities mentioned above. Is this default mode of human nature? How does our brain extracts "creative things" that we are never aware of in diffuse mode & where they come from?
[0] https://abraininitiative.wordpress.com/2014/10/29/focused-vs...
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4049052/
Not sure about driving...maybe road-rage adrenaline?
Yes. I think it's because when walking or showering some attention is kept in the body. This matters for thinking purposes because new ideas signal their presence at an emotional level.
Whereas sitting at a desk one's attention is mostly in "in the head" and this is also why lengthy sitting at work or in front of the TV is unhealthy.
Why?
I may state the obvious, but it's about the blood moving inside the body and to the brain and such, not about mere movement of blood, body and all along with the vehicle to some destination.