For something between standing and sitting, you could use a stool as I do. I have a bad back and I find it to be better than any chair I've used so far, though I've only tried a few. You still get a slight bit of elevated heart rate and caloric expenditure to keep your body firm and in good posture so I'd say it's better than sitting and more relaxing than standing. You also don't have to remind yourself to get up and move every once in a while because if you're practicing good posture, you won't be too comfortable.