Another option is a stool. Having the option to stand can make a big difference, even if you're sitting most of the time.
In my current setup, I stand between my desk and the wall. I can occasionally lean back against the wall to give myself a break. Between that and a gel mat, I feel pretty good.
Not just your feet, your knees, your hips and your back. I can't stand in the same place for more than 10 minutes before my knees and back start hurting. With a good chair, I can sit all day without problem.
Folks with weight problems or diabetes tend to have foot issues as well - exaggerated pronation/supination, plantar fasciitis, and swelling to name a few. There are solutions for nearly all of these problems and are highly recommended for general use, not just for standing desks.