A coworker used to sit on a giant exercise ball all day at her cube while she worked the sales phones. I'm not exactly sure what this accomplished, aside from making her look dumb. However, it would look less dumb than doing low lunges in your cube. And you're less likely to hit your head on your keyboard.
Actually, I think the exercise ball thing works your core. I'm going to look into that. It's better than a chair and I doubt they'll allow me to get the treadmill desk.