Bath bombs make bubble baths possible, so obviously they're awesome. They're fun for yourself and can make a great gift. They're also a little expensive if you pick one up in a nice store, but you can make your own for much less money.
Instructables user Rye Soap Kitchen puts together a nice tutorial on how to get this done. You need a few tools to do this yourself -- like kitchen scales, mixing bowls, a spray bottle with cold water, a circular mould of some kind, and some latex gloves -- but they're all things you already have or can pick up easily. As for what makes up the bath bomb itself, you don't need much. 300g bicarb soda, 150g citric acid, and a carrier oil (e.g. sunflower or olive oil) is all you need.
You can add fragrances and colour as well to make them more fun. If you want to create a bath bomb, just check out Rye Soap Kitchen's full instructions over at Instructables.
How To: Make A Bath Bomb [Instructables]