We raised rabbits when I was a kid, they are probably the most beginner friendly livestock.

As far as calories gained for effort expended - it depends on where you are. Fishing is very high calories/effort and so is deer hunting (depending on where you live) My daughter and I can legally take 12 deer each, per year. I like deer meat but the fat is not as good as beef, so I also buy beef. 6-8 deer is enough for a year since I also eat beef.

The bones are also great for bone broth, and of course you get 2 kg of liver and .5 kg heart per animal.

Also low maintenance fruit like figs and blueberries are much easier and less time consuming than tending a garden.