http://letterblock.com/images/treadmill_desk.jpg
So obviously I can do walking at various speeds and inclines (usually 2 mph at 4 degrees) or standing. I have an ergo standing mat that I put on top of the treadmill if I'm going to be standing for a bit. Whenever I get the rest of this office cleaned up enough, I'll look for a small easy chair for occasional resting or iPad use. Off camera there's also a stand fan, which you may need if you walk for hours and your office window faces south in Florida. (My office has always been hot, and I find that moving actually makes it more bearable.)