I bought too much G-Fuel and have been using it to make carbonated energy drinks. I may buy another brand of energy powder when I run out of my current stock. I also bought concentrates, citric acid, and sodium benzoate from https://www.heavenlysnowcones.com/ and Acesulfame Potassium from https://pforlife.com/acesulfame-potassium-ace-k-200x-sweeter-than-sugar-kosher-semi-natural-granular-sweetener.html that I still need to get around to using.
I have carbonated various other things before, including lemongrass I grew in my garden to make lemongrass soda, but it was too sickly sweet and the recipe needed to be tweaked. I bought a grape-like concentrate from a local African store that I didn't care for too much. I bought syrups from a local Bosnian store that were decent. I have used Mexican drink mix powder. Crystal Light packets were pretty good. I have also just drank carbonated water alone, and my wife mainly makes carbonated water.
There's probably more I have experimented than I have mentioned. G-Fuel powder tastes really good when carbonated, but I may not recommend G-Fuel as they are not really an ethical company, and their powder has a weird composition that doesn't really do a good job for energy and concentration. You can be very creative with your carbonated drinks, and you are not limited to the syrups that are typically sold for the sole purpose of being used with SodaStream bottles.
Also, I would love to figure out how to grow and use miracle berries as a sweetener than sugar or a carcinogenic compound to make sweet and healthier carbonated drinks.
