Yogurt is also extremely easy to make - just put a spoonful of yogurt in a smallish container of milk, and wait. It's also pretty easy to add things to, just like kefir. Fresh yogurt often tastes way better than storebought, and you know the bacteria are active!
I usually make 1.5kg of yoghurt that way, and it keeps for about one to two weeks — but it never survives that long (2-person household), so I make yoghurt around every three days. We use it for breakfast cereals, yoghurt with jam (home made jam for added benefit :-) and ayran. It's fantastic!
It is best prepared with just a couple of tablespoons of yoghurt (3-4) in a liter-sized bottle of water, with a teaspoon of (ordinary, white) salt added to it. Shake well, enjoy :-)
But I found it difficult to get a thermal pot down to the right temperature range to begin yogurt and switched to making only kefir. Kefir is less likely to lose to a contaminant at room temperature as it is a diverse cocktail.
I let the milk cool naturally to 115F, then I mix a tablespoonful of the warm milk with two tablespoons of my starter yogurt (for a gallon of milk). I continue to mix a tablespoon of milk into the starter until the starter is pourable. It then gets added to the pot of milk and stirred. If I'm making skyr, the rennet goes in at this stage.
The pot is usually at 112F-114F and get wrapped in towels and thrown into a warm oven, I usually heat the oven to it's lowest setting (200F) while the milk is warming, then turn the oven off while the milk is cooling. The milk sits in the oven overnight. The 105F-115F range is the "ideal" temperature for the bacteria.
In the morning, the curds get sliced, strained, and jarred.
Once you get used to it, making yogurt once a week takes an active 30 minutes of my time.
I believe that traditional skyr is made from bacteria in baby goat stomachs; the rennet is substituting this. Additionally, skyr is usually pretty thick, so I strain the curds until I get softened cream cheese consistency.