A few months back, I enjoyed assembling the Gakken Karakuri Somersault Doll (I got mine online from the Maker Shed). What's amazing about the doll is that it slowly completes a series of backward somersaults due to its internal mechanics and the potential energy it has from starting on the top of several steps. The mechanism was designed long ago during the Japanese Edo period. The hardest part of the build is probably threading the very fine filament which helps the doll's feet kick over the top during the somersault. There's also some tricky tuning of the internal weight of the robot, filament tension, and making sure that the costume doesn't inhibit movement. The pedestal spacing is also important along with the stand's placement on a solid surface where it won't slip during doll movement.
Even the knots on the acrobat's head had to be carefully placed to ensure the somersault goes smoothly. ;-)
And here's a video of a successful set of backward somersaults.