Depends on what you are looking for. If you want relief from soreness, Epsom salts are the best. If you want fun and relaxation, a lavender bubble bath would be nice. You can buy something like that in any CVS or other drug store. If you want to soothe dry skin, a bath oil with aloe in it followed by massaging in some moisturizing cream would help.
Check the directions to make sure it doesn’t warn against certain products. If it actually has tubing and a motor to create bubbles it might say don’t add anything that can clog the tubes.