I’ve been a little hard on kefir. At first I didn't care for the sour, yeasty tasting milk made from funny little grains. But one taste of pancakes made from the stuff and I was hooked! And kefir is very simple to make from scratch.
Water kefir is another probiotic beverage that can easily replace unhealthy soda. It is very simple to make and can be flavored in a multitude of ways. A second fermentation is great for increasing carbonation.