Relay race where the kids have to fill up a leaking pipe, using their hands to stop the water leaking
Drainpipes (50cm each)
Rubber drainpipe end (B&Q)
Drill a few holes in the drainpipe
Ideally so this near a stream - kids fill up buckets from the water supply, run to the poor wet person holding the pipe, and fill up the pipe. They'll need to cover the holes with their hands to stop it leaking.
When the pingpong ball floats out, they've won!