http://www.amazon.com/dp/B006FFR3FO/...6Z34R64C&psc=1
This is a good way to help keep tone and good posture while in the office. It doesn't necessarily provide a work out, but you will be using your core for support through out the day.
If you just want to keep the blow flowing through your day at the office, go for a walk around 10 am and walk for 20 or so minutes. Then at lunch walk again, and again at around 3 pm. Based on the average 8-5 job that should keep you lose.