This book has a number of stretches and exercises in it, many of which you could easily learn and then a couple times a day you could get up from your chair and go through a couple.
https://www.amazon.com/Posture-Straight-Younger-Pounds-Thinner/dp/1931945624
My boss got a copy of this book and put it in the break room because lots of his staff sit hunched over computers all day: https://www.amazon.com/Posture-Straight-Younger-Pounds-Thinner/dp/1931945624