I'm sure I'll think of lots of things later but right now for me it's braid a six-braid loaf of bread.
I've been wishing I could for decades. I once attended a King Arthur demonstration where the instructor made it very clear. I went home and I was able to do it with yarn pieces. But I didn't try it on dough fast enough and I forgot it.
I've got the KA cookbook that has pictures. I've checked out every other demo and tutorial I could find. I just CAN'T do it.