Find a largeish plastic pole, approx a good 10cm or so in diameter and slightly taller than the average Scout. Find and fit an end to it (your local diy shop). Drill some holes at random intervals from top to bottom, only do a few at first – you can always add some more at a later date.
Drop a cork into the bottom of the pipe; the challenge is to float it out again. The Scouts will need to block the holes with their fingers etc. Water can either be through a hose or better still a bucket and a handy local stream to get the water from.