Standing up burns more calories than sitting does and can be better for posture. Instead of emailing an officemate, walk to their desk. Stand up while you are on the phone, your counterpart won't know the difference.
Taking it a step further, if you are in a managerial role, try having walking meetings. If you can (i.e. the meeting doesn't involve taking lots of notes), walk around the building either within the halls or outside if its good weather.
I'll admit that at the end of long days where Ive been teaching classes and standing up training there are moments where all I want to do is sit.down.for.one.second.
But I would go crazy if I had to sit all day. I have way too much energy for that. I would definitely have to invest in one of these awesome treadmill desks! I think they are so awesome!!