Diy Cardboard Slide ~When I was a kid, we did this! ***One improvement: make the cardboard about a foot narrower than the stair width, so that there is space for the kids to climb up on the stairs rather than the cardboard slide.
Make your own Sponge Water Bombs (alternative to water balloons). Cut sponges in strips crosswise, then stack 4 wide, 2 deep and tie very securely in middle with fishing line. Soak, play, have sponge water fight!