I bottled 20 litres of kombucha yesterday with ginger and lemongrass. It'll be very fizzy and ready to drink in 3-5 days. Costs next to nothing and quite healthy - water, black tea, sugar, (gifted and self-reproducing) scoby. The flavourings are what costs most, depending on what you use.
Oooh that sounds good! I’ve been meaning to add fermented stuff to my diet.
Kombucha is the best "natural" soft drink for me, too. It's not entirely sugar-free, though (even though you can get it to low-sugar with longer fermentation).