I've had good luck (on the chemical free front) with using coconut oil and cocoa butter rather than commercial scented bath salts in the bath (my cats will drink from the bathtub and lick bath oil off me, and some essential oils, including Vitamin E oil, which is added to a lot of bath oils, can make cats sick). They make it harder to scrub the bathtub out, though, because they melt in hot water but congeal solid again when they cool.