Child’s pose is calming, easy on your lower back and offers a nice gentle stretch for your neck, spine & hips. Here’s a great guide from Yoga Journal to get you started! You can even rest your head on a cushion or block if you’d like.
Do it when you need to take a minute to focus on your breath and take a break from your computer screen. It can be done in almost any kind of clothes (but do take off your shoes).
Consider a modification if you have sensitive knees – you can modify the pose by putting a blanket underneath the back of your upper thighs to lessen any discomfort.