A major factor of fitness at work is nutrition. If you're interested in fitness at work, chances are you have a non-physically taxing job and would like to lose some weight to become more fit. However, the essence of work is that you're busy doing something besides exercising. About 45% of people gain weight at a new job that reduces or limits their physical activity. Since the technology boom, there has been an anti-boom in overall fitness.
What kinds of food should I be eating, you ask?
First off, you should be drinking water. Many offices have a water dispenser. Not only that, but you are often free to have drinks at your desk or cubicle. Sedentary working conditions lend themselves to heavy drinking, as there is often a small amount of time between tasks that is filled by nothing... just long enough to grab a quick sip.
Most work places also offer the use of a microwave for lunches. Unfortunately, the state of frozen foods is that the cheapest is also usually the worst for you. I suggest looking for a meal with less than 800 milligrams of sodium and 15 grams of total fat (remember to combine servings to reach the total).
Your best best, to improve your nutrition or fitness at work, would be to make your lunches at home the night before you go to work. This allows you to save money on prepacked food, and improve your diet with higher quality foods. Invest in some good containers and you'll save yourself a lot of trouble. Fitness at work can be troublesome, but it's worth it.