Kombucha is my drink of choice, but it is fermented tea so if tea is out so is kombucha.
The other options I can think of are are flavored seltzers/flavored water, Mocktails/virgin cocktails/Shirley Temples, agua fresca, non-alcoholic beer, and herbal tea. Availability varies, of course.
Nutritionally, kwas is just an eccentric kind of pop marketed to eastern Europeans. The modern mass-market stuff is even made the same way: syrup and forced carbonation.
