Go to the hardware or garden centre and buy a roll of black plastic – ideally 2 metres wide then set it up on any slope and run a trickle of water from the top. You can cut some squares from the roll to use as slide seats – this reduces scratches and damage to the turf. Add a dash of dish washing detergent every now and then for a really fast slide.
The grass at the bottom of the slide will take a bit of punishment but the nitrogen in the detergent will usually result in a fast recovery.