We make simple bath salts in school, usually for mother's day. You just need epsom salt, fragrance (skin-safe fragrance oils, essential oils, or we have even just used perfume sometimes) and if you want different colors, a little bit of food coloring.
Pour the epsom salt into a bowl, add a few drops of food coloring - and a few drops is all that is usually needed, depending on how you want the color, and add the fragrance to your desire. Mix well and store in an airtight jar. We used baby food jars, but you can use any jar. They are quick and easy to make.