Balloon ninjas to shoot with nerf guns. This is a fantastic idea! Great for little boys! I know this was posted as an idea for little boys. but I think I'd like to have a nerf gun battle with these ninjas.
Glow in the dark egg hunt. Could spray paint the eggs with glow in the dark paint instead of using glow sticks. Also, doesn't have to be for Easter. Could be a fun night time scavenger hunt or other fun summer game.
Color Hunt - practice colors and play! Each kiddo could have a different color of construction paper and they have to find all the objects that are that color. and then we can dump everything back in the middle and switch papers!
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.