A simple and quick bath additive. Put 2 tablespoons of oatmeal (preferably the fine ones) and, if desired, a few dried herbs in a tea filter. Close the whole thing with a clamp, for example, and simply add it to the water when you fill the tub.
While bathing, gently squeeze the filter bag from time to time to distribute the oat milk and herbal extract throughout the tub.
If you want a nice gift, you can also sew the bags closed with a sewing machine or by hand.