I like the sounds of the some of the suggestions so far, especially lime and lemongrass cordials.
About the only sweetened beverages I'll drink are cane sugar Coca-cola and real ice cream milkshakes, so adding sugar and little else to plain water -- still or sparkling -- is not my custom.
For still water, when I'm away from home, I've recently discovered
True Lemon, True Lime and True Orange packets are a winner. Sparkling water I can handle unadulterated, but I do quite like a healthy dash of
Angostura Aromatic Bitters and a slice of lime whenever possible. Tastes divine (and pretty, too)!
ETA: I wish Angostura came in 4ml mini-bottles, like Tabasco Sauce does.
