This sounds like a good science fair project.Soda sink or float. Have kids predict whether sodas will sink or float. Most will assume they will all act the same but they won't! Diet pop floats and regular sinks. Nice way to touch on sugar content too :)
St Patricks Day Sink Float Experiment Science Activity
Floating and sinking is a fun science concept for children to explore. Setting up a Floating and Sinking Learning Tray will invite them to play, experiment and explore how and why different objects float and sink when placed into water.