Here is a gorgeous motorized marble machine with 8mm marbles that can keep you mesmerized for a while. The Serpentine is made of a solid 2cm thick beechwood. The balls roll down on copper tracks which are bent and soldered by hand. They go through 16 turns before working their way back to the top.
This marble machine measures 12 x 10 x 10cm. You can find it on Etsy.