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.
Balloon Rockets - I've done this in several science camps I've conducted. Kids have fun with it. But, when the balloon deflates it may fall off the straw because it's surface area changes - another good thing to show and explain to kids.
Marble Races with a cheap pool noodle (occupies the kids for hours.) Did this, but instead of cutting the pool noodle, the marble is small enough to fit inside the full noodle to roll down. Then can use the noodle for other things too!