Interconnected Arduinos via digital pins and magnets
While idea looks cool, you need some kind of "locks" because not "any combination" can work. 

Rotate second Arduino in bottom row on 180 degree and you will see that GND/5V reversed relatively to first arduino in second row.

You have to account magnet N-S polarities to allow good lock. Probably you can use magnet polarities to achieve proper orientation.