A little bucket (small enough for the child to carry), some bubbles, a sponge, water and a cloth. They can wash just about anything (think toys, outdoor furniture, gardening tools, bikes). It really helps if the item is dirty/dusty/muddy, this makes the work purposeful in the way the child can visually see their work of cleaning. 

I demonstrate to Otis three steps. Wash. Rinse. Dry. But of course there is a whole heap of mischief and splashing in between. 

When demonstrating and working along side Otis I pay attention to detail. Washing every last part of the item, cleaning into corners and crevices. While this looks like fun play it's also work, making sure the engine is clean and sparkling, using fine (with sponge and cloth) and gross (carrying and tipping bucket) motor skills and building concentration. 

The reward is instant. A nice clean fire engine.