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 Sand and Rock Box by FSPDT. Gdjk -> made this with toilet rolls (aka tunnels) and moon sand instead of play sand. My son loved playing with it. The moon sand cleans up nicely indoors but play sand will be easier to play with when using the small cars.
Summer games wouldn't be complete without some water balloons! Pour water into balloons to create a water balloon board! Your friends and children will have fun aiming darts to pop each balloon as it explodes with water.
Backyard Kerplunk Game: The goal is to get out the sticks without any balls falling out. Children and adults will have a blast challenging each other with this. (via All Outdoor Games to DIY This Summer via Brit + Co.