Go to the lunch room frequently to make a cup of coffee or tea. Any by doing this frequently you'll have to get up later to go to the restroom. See, two exercises for the price of one! A little more seriously, I've found a number of benefits by making it a priority to get out for a 20-30 min walk at lunch every day. If your building has stairs and is large enough, go for a 5 minute stroll once in a while.