In Family Fun magazine August 2009, there was an activity called Zoom Flume.
"Transform your ordinary backyard slide into a water park wonder by adding a steady trickle of H2O and a soft off-ramp that extends the ride. To make one, use duct tape to attach a garden hose to the top of the slide. Place camping pads (or other cushioning, such as lawn-chair pads) end to end on the ground at the bottom. Stretch an 8'X10' or larger tarp across the pads and place tent stakes in the tarp's grommets to secure. Note: Because water can make slides extremely slippery, parents should provide supervision to keep play safe."
Since we have a slide and it gets a little *ahem* warm here in Texas, we gave this one a try. Instead of camping or seat cushions, I used Kindergarten nap mats—we had two around anyway from naptime and sleepovers—and these already have a vinyl cover. Instead of a tarp, I used our Slip-n-Slide. I attached a sprayer to the hose and instead of a steady trickle, I randomly sprayed my daughters and the slide to make sure everyone and everything got good and wet. They love it and ask for it again and again!
WARNING: Do not leave hose unattended or innocent bystanders may unintentionally (or intentionally) get wet.