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.
The Pinspired Mom: DIY- Noodle Horses/Pegasus (Flying Derby Ponies! You could use these pool noodle ponies for a unicorn, My Little Pony or horse themed party. Cardboard cones could be added to make a unicorn
King's Music Room: Making Swimming Noodle Ponies. This activity is 10 kinds of awesome because finally my kids will stop using these noodles as swords. Yes. sword-fighting skills are natural for preschoolers and toddlers.