I think just plain water or you can get a soda stream and make your own much healthier soda. as you control the syrup input etc.
reminds me of that movie where a deer pisses in a spring river and it follows the river all the way until someone is drinking spring water.....forget which movie but it might be leonard 6 because i know it has a similar opening but not sure if thats the one. edit: no leonard 6 is with a killer fish going through the water to a pool or something.
There's a firm here called Green Bottle that does a ginger and elderflower cordial. I will get some when I next order my grocery delivery. They're doing a deal on it. You mix it with fizzy water.