The push-back push-up forces your shoulders to work hard to drive your body back using a natural arc and push angle.
Just drop down, explode straight back, and return to push-up position. To take it up a notch, put your feet up on a bench. Your abs will be begging for mercy too.
A 2-kilogram discuss traveled 140 feet through the air and landed in Dan John's skull. After that, he "rested" for five months. Now he has some interesting ideas about rest periods in general.