Try extracts. They add flavor without calories or sweetness. I like licorice(of course), and peppermint. Doesn't take much to flavor the water. Just a hint of flavor is what you want. Too much is too much.
Licorice sounds divine. Is there such a thing as carbonated licorice water?
I answered my own question because I don't like to be spoon fed answers... =-D
LICORICE SODA
3 cups water
2 oz. dried licorice root, chopped
4 star anise
3 cups brown sugar
seltzer water to serve
Pour the water, licorice root, star anise, and brown sugar all in a large pot and bring to a simmer over medium heat, uncovered. Let simmer 5 minutes, then remove from heat and allow to cool to room temperature. Strain through a fined strainer (I store mine in a canning jar) and chill in the fridge.
Store in the refrigerator up to 2 months.
To serve, ladle about some syrup into a glass, and pour seltzer water over syrup. (We liked about equal parts, but play around and see how strong you like it.)
Makes about 6 servings, but depends how strong you like it and how big of glasses you serve in.
This might also work. It's a syrup premade for flavoring things...
Or
Gross Gus's Dragon Drool black licorice soda. Might order some and give it a try.