At the end of the day, sometimes I have a hard time getting my mind off of work. Physical activity usually helps me clear my head.
In a pinch if there isn't time to work out, this helps: a wide-knee child's pose with my head resting on a block or the floor.
The key is the gentle pressing of the forehead into the floor or block.
Basically, with this simple gesture you're telling the intellect that it's time for it to rest and turn off. It has done a great job today and now it gets some time to kick back and relax. It's kind of like a reset button for the mind.
I usually do my wide-knee child's pose on a folded up blanket so it is easier on my knees. Sometimes I put a throw-pillow from my couch under my abdomen or chest and then my head on a block or pillow. Sometimes I let my abdomen and chest sink freely and have my forehead on the floor.
If your knees are still uncomfortable, try a rolled up hand towel behind the knee (in the "knee pits") to create some space in the joint.
Once in the pose, I breathe deeply and feel my lungs expand my back body. Sometimes I gently rock my head from side to side, massaging my forehead. I might stay 3 to 5 minutes. I feel my lower back relax, a gentle stretch along my spine, my inner thighs get a stretch.
When it is time to come out of the pose, do so slowly. Walk your hands in and use your arms to push up so you are kneeling. Take a few breaths. Then use your arms to help bring your knees back to center from their wide position. Take a few breaths. You might lean forward into a "table" position and do a few cat-cows and a gentle bent knee down dog where you "peddle out" and stretch your legs.