The one we make and love (and it's sooooo easy) is juice kefir. Our favorite is grape juice kefir. Just take milk kefir grains that you don't care about keeping to multiply, throw them into some juice for 12-24 hours, and you have a fizzy delicious fermented drink.
W/ the grape juice, there is SO MUCH sugar in grape juice that it is still sweet even after 24 hours of fermenting--it tastes just like 'sparkling grape juice'. It's a ferment both ds and dh love! All visitors love it as well (although I'm sure to give them just a small amount--wouldn't want to greatly impact anyone's digestion the first time they drink it!!)
We've made pomegranate juice kefir, apple juice kefir, and coconut milk kefir as well, all w/ the same milk grains (they've lasted me over 6 months w/out being returned to any milk! I think I'll need knew ones soon--they are fermenting a little slower now.)
ETA: If you leave them in the fridge for too long, they will continue to ferment a bit and taste kind of alcoholic. I've certainly felt like I was taking a shot at times when finishing the last bits of my juice kefir--just watch out for that--it can be a shock!
