If you have a hill and access to a water hose then the Wahii WaterSlide is an activity that the kids will love. It\'s 75ft long x 12ft wide. The kit comes with fasteners that hold it to the grass and set-up takes about 15 minutes, but will decrease with the kids helping.
The cost is relatively inexpensive compared to renting or buying a commercial grade inflatable water slide. Plus inflatables slides are usually not big enough for older kids (10-17yrs).
Check out Amazon.com for reviews and how other church groups, youth groups are using it. You can buy it online.