Ditch the chair and get yourself a stability ball. You can get them for like $8 at Target or Walmart. Just be sure to get the proper size for your height - just go by whatever it says on the box. It's a great way to burn calories while sitting! It will also help improve posture - and you know what they say right? Something about how proper posture can make you look 10 pounds thinner or something like that?
I sat at a desk for over 10 years. Changed careers. I'm a personal trainer - which means I don't get to sit. Ever!
I can put an average size man on my back, squat, get up, and do that oh about 10 times or so. Rest and repeat.
I've been working on a couple of websites for the past few weeks - which requires waaaaay too much sitting and I end up sore and stiff. After a few hours I can barely move. OK well, I'm old too. But still - it's physically more painful than my most intense workout! Sitting for long periods is killer on the body - so much so that you'd think it'd burn more calories than it does!
Oh or you could try those pilates resistance bands for while your at your desk. There are some upper and lower body things you could do. Really though, make sure you get up every 20 minutes or so to stretch if you can - not for the calorie burn but simply to save your back, neck, knees etc. Too much stiffness/tightness usually leads to things like back, hip, neck, and knee pain.
My back (and chiropractor I'm sure lol) thanks you for this! You reminded me that I keep meaning to ditch this chair!!! I've got a stability ball downstairs I keep forgetting to bring up here!