Imagination is not optional for intelligence. Intelligence is the ability to build mental models and manipulate them.
I like this thought. Not quite sure what counts as imagination though. Does the ability of a chess algorithm to model hypothetical future board positions count?
My experience - writing a very simple rubik cube solver as an undergraduate project - I rejected the two simple solutions for a trivial case (requires 1 turn to solve). So it turned the opposite face, then turned the first face, then turned the opposite face back. This had the appearance of a creative solution even though the algorithm was dumb.