An inflatable pool makes a great safe play area for babies and toddlers. This website has some really good ideas! wonder how long it would take miss priss to get out of this! Good idea though if you have room for it.
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.
This could be a fun science fair project someday :) Ants stomachs are transparent. a woman noticed it when ants drank/ate spilled milk one day so she gave them some colorful candy and then photographed them!
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.