somewhere for the children to climb
a boat/pirate ship to imagine sailing across the sea in your lunchtime
a quiet area
living willow sculptures are lovely
I think play equipment is equally as important though, elastics, skipping ropes and large jump ropes, learning the old clapping games and rhymes
Using the adults who supervise as resources themselves that instigate games and not just walk around talking
a large sandpit is great with buckets and spades not all children ever get to the seaside.
an area for curriculum gardening. I did a dig for victory garden with my year 4's and you would be amazed how concerned they were with the success of their plants (we were trying to grow a tomatoes for a sandwich for our head, they were mortified when their sweetcorn was snapped, they ever tried to repair it!)
A dirt pit that can be used (weather dependent) for cars, machinery, trucks and diggers.
I would also suggest that boiler suits be worn so children would be free to play, explore, have fun and experiment without the worry of getting their uniforms dirty. (we have these at ds's school)