I have pretty much stuck with real lemon, real ginger, dried organic fruit or raisins in my brewing water kefir and had great success. Once I put the 'finished' [two days for me] water kefir into plastic soda bottles for the 'second brew' I only have ginger for extra flavor while it's getting a second FIZZ. All this is done WITHOUT refrigeration. In fact, I never put it in the frig. I love the fizzy warm healthy drink right from the cabinet!
Once harvested you can add whatever you want to it! Sweeteners like stevia, fruit juices, herbs, whatever to your personal taste. Just remember NOT to add these while it's 'brewing' as it may alter the brewing process.