Cast your mind back to the toys of your youth. The Jack-in-the-box, the Slinky, the pull-back-let-go racing car, the action figures. They had an inherent playfulness at their core. Things to touch, feel, bend, and stretch.
I remember the interlocking latches and spring-mounted, slide-out parts of my Transformers action figures. They offered a user experience that required no goal setting and no difficulty curve. The simple, tactile pleasure of positioning and repositioning their moving parts was reward enough.