There are quite a lot of exercises you can do in an office chair or standing by your desk. But don't discount other work spaces.
Sit, stand, repeat x10 before you actually sit and do your biz on the loo. :)
Do calf drop/raises in the stairwells.
Don't multitask. If you have to drop off a file and get some more Twink and also go to the loo, do them in sequence, not on one trip. More steps...
Deb, in New Zealand