If bitters are acceptable to you, you can bring those on board in small quantities in your liquids allowance and add them to the onboard soft drinks. Alcohol content of a soft drink with a dash of bitters is going to be > 0 but < 0.1%.
There are also powered soft drink mixes of many sorts which you can bring on board and mix with water (still or sparking) on board. Check out the "wellness" (i.e. hipster diet and exercise) section of a supermarket or health food store. Various manufacturers now sell water-infusion bags similar to a tea bag to be placed in a water bottle to flavour water.