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!
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.
Paper plates, Popsicle sticks, and a balloon are all you need to play Balloon Ping-Pong. (Not sure how sturdy the paper plates would be, but may be able to do something similar for a carnival/game night social event theme.