I made some candles as Holiday gifts for may neighbors.
Here is a slideshow with links to materials and a video talking about making them.
4 ounce candles end up costing about $4 each to make using soy wax and essential oils.
I think it would be a good school project, or club project (once you can figure out how to handle the cost). There is much science and math involved, as well as social studies/history talking about the various essential oils and what parts of the world they come from or what role they play in various cultures.