Traditionally we used buttermilk and whey here. Also barley water.
Make some crowdie/cottage cheese and ferment the whey from it. No waste and all good food
Barley water is easy and is usually drunk on the day it's made, but if you let it sit cold it ripens. It 'can' thicken up a bit too like the kefir. Just give it a good stir. If you haven't let wild yeasts into it it shouldn't ferment, but not a bad thing if it does, ends up mildly fizzy, If it goes mouldy though you left it too long, throw it out.