For summertime fun, you got to have a Slip 'N Slide. There's even an easy way to make one too. Make your own Slip 'N Slide using plastic tarps. A store-bought Slip 'N Slide is just a cheap skinny tarp anyway. Get your own tarps at Home Depot, and poke holes in a cheap garden hose. Then run the hose along the edge of the tarps to keep them wet.
If you have a big enough hill, you can string tarps together for a REALLY long ride. To make it faster, dump a few bottles of baby oil on the tarps first. For less than you pay for a real Slip 'N Slide, you get one that lasts longer and gives a much better ride.