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.
Make these simple sensory bottles as a hands-on manipulative to talk about the weather. Great for circle time! I would use this in a kindergarten class. put the children in 5 groups and let them tell class mates what they think about their bottle.