Here are a few ideas. You may need more than one Slip ‘N Slide.
Slip ‘N Slide Bowling – Buy some inflatable bowling pins to put near the end of the slide and compete with each other to see how many pins the kids can knock down as you slide.
Jousting with Water Balloons- set up 2 slides if yours is not a double slide. As kids slide at the same time, but past one another, they get to throw water balloons at each other to see who gets the wettest. No jousting lances involved.
Toss a Sponge – Kids carry soft objects like nerf balls or tennis balls and see how many they can get into the targets. Use cardboard cutouts, plastic pails, inner tubes, laundry baskets as targets and set them up along the sides of the slide. Then let the kids slide.
Targets can be anything with a hole in the middle that is safe.
I hope that helps. There are other examples out there. Google can help.