I got a height-adjustable self-righting mushroom stool that can quickly height adjust. I found that finding the right height setting is a good intermediate between standing and sitting. Almost standing but definitely leaning. I can stay in this position all day but it's not particularly comfortable as either sitting or standing. I sometimes lower the mushroom stool but it's least comfortable for sitting, too much concentrated weight pressure. I thought I could use a huge yoga ball but glad I didn't that wouldn't have worked as well.

I keep a small wooden chair around for if I'm really too tired to balance. But it's not comfortable either so I don't use it unless I really need to. Wearing slippers seems to be enough, but maybe I'm not noticing/attributing any back pains to it.