a water slide made from tarpaulin
big tarpaulin, wasking up liquid (LOTS), hose pipe
beavers slide along tarpaulin on soapy water
For scouts: use 25m+ of strong builders plastic sheeting, preferably on a campsite with a slight downhill. Use hike tent pegs (hammered down to ground level), pushed through a folded-over edge of the plastic, to hold down edges of tarpaulin. See who can slide the furthest from the start. Long run-ups work well :-)
Have towels around for smaller scouts who may get cold quickly, and water to wash people's eyes/faces if they get covered in soap suds. Check the ground for lumps/stones first...