Forklift
A long-planned project that I never had a chance to try, the forklift was to be an ambitious animation. The idea was to have the forklift unload an object from one location and move it to another. Then, reverse the sequence (some time later). The object that the forklift was to move would be a free object, not something attached to the truck or to a mechanism. The forklift would traverse a path that would move the object 90 degrees—say, from a rack to the floor, or from one rack to another. Also, the truck would back away from each move to show the object is not attached to the forks. View of the animation would be somewhat restricted owing to the fact that the floor or ground would have slots in it for the mechanism.
This would require quite a complex mechanism, involving all manner of cranks and cams, limit switches and whatnot. I'd still like to do it just to answer the challenge.