Bath bombs require only a few ingredients. You can add custom scents and oils to make a soothing bath even more enjoyable. Some of the ingredients, such as baking soda and food coloring, may already be in your kitchen, but you might need to go shopping for others, such as citric acid and essential oils.
Hard water? Citric Acid makes life much easier when it comes to cleaning build up or stains from hard water. I put it in my dishwasher w/ my detergent, I use it to ungunk my faucets, a little vinegar added to it and it makes a great cleaner/paste for tub scrubbing, etc. It's not harmful - edible in fact - and has many other uses. If you use Lemishine - you could have this for the price of 5 small containers.