Use recycled juice, soda or water bottles, a filler of some sort (dry or wet) and a discovery material. The bottles above have mineral oil, water, food coloring, and beads or cut up straws. Keep in mind that liquids will always separate from oil. Super glue or hot glue the caps on so that they can't be opened when being played with.
Wet fill suggestions:
clear/tinted shampoo or body wash
Dry fill suggestions:
Discovery items can be almost anything imaginable: