Why not use solutions with density greater than one? Make solutions of Kosher salt in water of various concentrations, up to near saturation. A little food coloring of the various concentrations can help to distinguish them, and students can be challenged to "stack" the various colors in a transparent or translucent soda straw. This takes some dexterity because the solutions are miscible, but it is an active learning challenge. [Kosher salt makes clear solutions, whereas table salt does not, because of a polymer coating to prevent clumping in a shaker.]
