Students squeeze the bottle to make the eye dropper sink. A second bottle has a challenge in which students try to link a diver with a hook into another hook. Two immiscible fluids with coloring. They compare the speed of the waves that develop. In the bottle with two fluids, the densities are nearly equal, which reduces the effect of gravity and slows the waves.
Water fills the first bottle entirely but other bottle only partly.
Students blow between bottles and see them move together. Two bottles, one filled with water, the other with water and some beads. Rubber bands around outside for traction, and a small ramp to race the two on. The beads inside one bottle are heavier than the liquid and take more energy to get moving. Therefore the bottle with the beads always looses the race.