I've also done this for Christmas gifts too. I usually use epsome salts with a couple drops of essential oil and a couple drops of food coloring for fun. Another idea is to put the bath salts in small tins that you can find either new at craft stores for little $ or antique ones at flea markets, etc. (I like to recycle old tins or jars). If you knit or crochet, you can also include a bath scrunchy or mitt with the bath salts.
This year my husband and I are making soap too, so everyone will get a homemade spa kit with: bath salts, soap, bath mitt, and eye bag (made with scraps of silk and filled with rice and lavender). Have fun!