A while ago I got a SodaStream water carbonator to replace my slowly-failing iSi siphon. While I haven’t used any of SodaStream’s own flavor syrups (as I have heard they are generally nasty, and all of them use sucralose which tastes bad and gives me a headache), I have of course made several bottles of soda flavored using Torani syrups. However, I found those to be a bit sickeningly-sweet, not to mention overpriced, but after a bit of experimenting I discovered a very simple means of making soda which is quite delicious and also much healthier: herbal tea!
Basically, after carbonating a 1L bottle as best as you can, put two herbal teabags in (I usually remove the tags and use chopsticks to remove them) and let it brew overnight. If the tea you selected isn’t sweet enough, you can add a couple tablespoons of simple syrup or honey or the like afterwards.
I find that Tazo’s herbal teas work very well; their orange blossom and passionfruit teas are way too fruity and sweet for use as a hot tea, but as a soda flavoring they’re perfect, and very low-calorie, too.
Stash’s lemon-ginger tea is also pretty good as a ginger ale surrogate, although that needs quite a lot of sweetener. (I’ve also found that it doesn’t hold its carbonation very well.) Of course, for ginger ale there are better recipes for brewing it (which involve actual fermentation).