Go.
Stairs.
Walking.
Eating celery.
Having conversations with stupid people can be totally exhausting, also.
Eating celery?
I’m guessing you are refering to the comparitive calories in stick of celery compared to those burned by eating it.
<goes off to seek out stupid people to talk to - doesn’t go very far>
I found this guy’s solution intriguing, but I don’t think I can afford it right now:
http://www.usatoday.com/tech/news/2005-06-07-office-fit_x.htm
I recommend fidgeting.
I’ve fidgeted for six years… I must be doing it wrong. 
Snacks at work…fruit, nuts, etc.
I find when I get the late afternoon munchies chewing a stick of gum helps. I’ve found the “5” brand gum to be very flavorful and long-lasting.
Water. Drink lots of water.
Use the bathroom farthest from your desk.
Along the same lines as the bathroom suggestion - if you have more than one printer/photocopier etc in your office - set them up to use the one furthest away.
Use your lunch and whatever breaks you get to take a walk.
Get a resistance band and do some exercises. Not huge sets but a single set of ten repetitions of two three exercises won’t drench the pits. Do it once every hour or two with different motions. Gets you out of the chair, tones you up, burns calories.
Over cook the microwave popcorn. The black stuff is burnt calories.
I haven’t personally tried it, but replacing your chair with one of those exercise balls is supposed to burn calories as your body core tries to keep you balanced and upright.
We’ve had this result in exercise for the whole building . . . when the smoke detector triggered the fire alarm.
It was an auditor who overclocked the kernels. Later that day his auditor buddies cobbled together an invoice, charging him for everyone’s lost productivity during the evacuation. Merrie fellowes, these auditors.
And it keeps your co-workers amused all day!
I’m using one right now. I also have one of the balance ball chairs (a smaller ball on a frame with casters) and a regular chair; I rotate among them depending on how tired I feel, what I’m working on (it’s hard to swivel on the ball), etc. But I work at home, so no co-workers to laugh at me.
I came in here to suggest The Hawaii Chair.
Like someone above said stairs.
For motivation make a spreadsheet which keeps track for you. Time your stair route and put the calculation into the spreadsheet so that you can see how much you have done. I am sure you could even find an approximate value for how much energy a flight of stairs burns.
Use larger fonts
Or, uh, don’t snack?
Depending on how concerned you are that co-workers are going to mutter about the “weird guy”. . .
Do pushups – I don’t know what your office situation is actually like, but if you’re office is semi-private, or you just don’t give a shit, you can “drop and give me twenty” any time of the day. It takes 20 seconds, gets your heart rate up.
Definitely take the stairs.
Find something to keep your balance and do deep one-legged squats.
Bring in a 10 pound weight, and just play with it all day like it’s a koosh or something (I basically do this at home).
But, I really don’t think you should have to worry about this stuff at the office if you’re doing enough outside the office.
Office chair race! Great lower-body exercise.
(Yes, I’ve done it)
Volunteer for physical tasks–moving things, fetching things, lifting things. Burns calories and makes you look good.
Have sex with a cow-orker on your desk.