I haven't been able to find anything very simple. Tannins might be safest, though, its in plants. The stuff that causes "tea water" in aquariums due to fresh wood added. And there's brain tanning, kind of extra gross. When I butcher them, I'm trying not to deface them all that much and feeling the brain is what makes the animal, messing with it is wrong to me.
I'm using Alum now, first time, very easy. Just dump them in the mix, scrape them, dump them back with a double mix and then break the hide.