Get one of those stand-up desk, then. Simply standing and getting up your toes can be quite active. There were quite a lot of discussion about these recently on reddit.
I am on the stand-up desk program and really like it. Though it isn't excercise from a cardio perspective, it can improve your posture (though like anything, you can still slouch at a stand-up desk).
I also sometimes stand on a balance board to make things a bit more interesting and engage core muscles while at my desk - though the second you touch your keyboard or mouse, you'd be amazed how much less you use your core for balance.
would love to, but part of the professionalism at the company I would for is identical desks ( I have asked if I could bring in my own desk, was told no, it would encourage people to be unprofessional)
I also sometimes stand on a balance board to make things a bit more interesting and engage core muscles while at my desk - though the second you touch your keyboard or mouse, you'd be amazed how much less you use your core for balance.