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.
29 Dollar-Store Finds That Will Keep Your Kids Busy All Summer
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.
A bar of Ivory soap .this is totally genius. And not just for kids! This is what happens when you microwave a bar of Ivory soap! Then you can tear it up, color it, and mold it into shapes! It's not wet and messy, either! This is awesome!