An easy activity for a hot day. Find a nice slope near a water source. Lay a long piece of polyethylene pastic on the hill. 100' long and 6' wide is good. Ensure it is secured at the top and there are no sticks or stones to puncture the plastic. We used a gasoline powered pump to suck water from a lake and up to the top of the slide. Multiple hoses were used to minimise pressure loss due to friction.
Alternatively, if no pressurised water supply is available, use a bucket of water and dish washing soap.