what are some exercises that can be done while sitting at a desk in an office all day?

I may have missed it, but I didn’t see anyone suggest a standing or treadmill desk. I use one for all the things I used to do sitting at a desk. I sometimes just stand at the desk, but most of the time I have the treadmill running at a slow pace. Pace really depends on the person and their familiarity with the desk set up. I do set these up for clients and teach them and my instructor prep course students in the use of the treadmill desk.