go for walks during your lunch/break... if there is an office that is either out of the way or closed off (i.e. no windows) you can do crunches, pushups, tricep dips and other body weight exercises... i do this in the board room once in awhile...
ummm i bring in a dyna-flex ball to work my grip and forearms...
best thing to do is really concentrate on your posture, it won't do a lot for your body composition, but it does wonders for the appearance... (most people with desk jobs tend to hunch over) if you concentrate on using your stabalizing muscles to keep your back straight (suck the abs in a bit, not holding your breath, but actually contracting the muscles)... this will also help strenghten these muscles which will improve your gym workouts in general...