Keep it simple, stupid (or smart) Before or after lunch you can exercise 15-20 minutes at low to moderate intensity. It’s not difficult. *Make note if after a heavy lunch, consider low speed walking only.
Do you have more than one floor in the build? If yes, climb stairs. climb up to the top, stretch a little then walk down for cooldown. If your knees hurt, take the elevator down and repeat (this may be a problem if your office is busy) other solutions as follows.
Do you have an outdoor parking lot? If yes, walk the perimeter of the space. Not in a parking garage, fumes and bad air ventilation is bad for you.
Do you have sidewalks around the block? If yes, walk around the block or a few blocks.
Consume the sunlight, burn calories and get the blood circulating again. Sitting for prolong periods of time are not good for you anyways. For me that’s what I do all day in the architecture profession (most office settings can relate). They have options to raise the desk so you can stand f you want, but I still feel out-of-shape when sitting there for hours. :-)
So remember KISS in the office! (or out)!