Volcano
Turn this project into an experiment instead of a demonstration (see the questions at the end).
Forces and Motion - Roller Coasters
Learn about the forces in action in a roller coaster from Sci Show Kids.
Paper Roller Coaster from Science Buddies
Pool Noodle roller coaster from Teach Engineering and Scientific American
Slime, Oobleck, and Elephant Toothpaste
Slime from Sci Show Kids and Science Bob
Oobleck from Sci Show Kids
Elephant Toothpaste from Science Bob
Color Changing Milk Experiment from Scholastic
Grow Crystals at Home from Babble Dabble Do (several different recipes to try)
Make a Rubber Egg from Babble Dabble Do (great extension questions/experiments at the end)
Density experiments from Science Sparks
Blobs in a Bottle by Science Bob (Lava Lamp)
Reverse Engineering - Take apart a toy (with parent permission and guidance) to learn how it works. Make sketches, learn about the mechanics, and then use the parts to make something new.
Take apart a push-toy from Teach Engineering
Take apart and rebuild new toys from Instructables
Hack a Musical Greeting Card (do you have one lying around the house?) into a from PBS Kids
Skittles Experiment (learn about diffusion) from Science Sparks (Don't have Skittles? Try other candies!)
Experiment with Capillary Action
Make a Solar Oven (cook a S'More)
Great video from PBS
Another design idea from NASA
What Makes Ice Melt Faster? (and other ice experiments) from Little Bins Little Hands
Weather Experiments
Cloud in a Jar from Little Bins Little Hands
Water Cycle demonstration from Gift of Curiosity
Make a Thermometer from Little Bins Little Hands