Just lurking this thread, but my (our) low calorie (and only, really) soda treat is Soda Stream with a slice of lemon, or not. Everything in the system is reusable and very inexpensive. And we love it. The one litre plastic bottles have a use by date some years in the future. Our old ones are great to freeze raw milk.
We hated the endless stream of waste plastic we generated buying from the soda water supermarket more than the expense. Those bottles do make some nice, inexpensive fleece jackets, though.