If you're sedentary, you could benefit from working stabilizer and core muscles, and this is possible without equipment.
Squats - even without weight - will strengthen your legs, back stabilizers and the connective tissue in your knee. Do multiple sets to near-failure.
Planks - get in the "up" position of a push-up and stay there, back straight, for a good 30-60 seconds to start. Repeat several times.
This is assuming you can do this while taking short breaks from your work. If you absolutely must incorporate it into your actual while-working time - I don't know, attach a 45 pound plate to your mouse or something.
Hopefully at some point you'll start to feel some benefit, and this will motivate you to dedicate some time to exercise outside of work.