Fidget in your seat. While annoying to coworkers, squirming and tapping can burn calories and even prevent them from turning into fat.
"[In one study where 16 men were given 1,000 extra calories for 8 weeks, some gained as few as three pounds, some as many as 16.] Neither the volunteers' basal metabolic rates nor prescribed activity levels could explain the observed differences in fat deposition among their volunteers. Instead, the Mayo Clinic researchers concluded that the differences in fat deposition must have been due to what they called non-exercise activity thermogenesis, abbreviated NEAT and known commonly as fidgeting. NEAT, the investigators explained, includes thumb twiddling, standing as opposed to sitting or lying down and other kinds of nervous twitching. They measured the calorie contribution of NEAT activities before and during the overeating trial ... and found that the volunteers whose NEAT expenditures increased the most during the overeating phase deposited the least amount of body fat."
From Why Calories Count: From Science to Politics (University of California Press) by Marion Nestle & Malden Nesheim.