It occurs to me that it would be wonderfully perverse to automate Matthias Wandel's marble-based binary adder. All it would take is a microcontroller and some sort of widget to place marbles in the input slider. A traveling gantry would be interestingly complex, but the marble feed might be overcomplex.
You could even add a marble pump to close the loop.
On the same topic, of cool mechanical things, my uncle forwarded a clip of one of db fletcher's expanding tables. This would also be neat to re-make, but, you know, expensive. Oh well.