For younger kids, let them experiment by testing whether objects sink or float in water. Let them guess their prediction before testing it out. Does a heavier toy sink to the bottom? What would happen if you put a paper clip in water?
For older kids, let them design a boat. The boat can be made from a number of materials such as paper, twigs, popsicle sticks, etc. Let me use their imagination. Then let them test their creation to see how many small objects they can put in their boat until it sinks.
Have fun experimenting!