Think you're pretty good clapping behind your back, eh? My boxing coach could do 20+ clapping behind his head AND landing back on his knuckles to perform the next pushup on his fists.
But if you want a challenge.... here's one for you:
Straight-arm pushups.
Lay face-down and extend both arms directly in front of you, and place one palm on the other on the floor. Make sure your toes are on the ground as you would for a regular pushup. Then, using ONLY your toes and palms as points of support, attempt to lift your body off the ground.
Once again, laying on your stomach.... both arms completely straight an in front of you. One palm on the other on the ground. Legs straight and up on your toes. Then pushing ONLY with your toes and palms (knees, forearms and elbows can't be touching the ground) lift your body off the ground.
You can try it at home. And be honest.... if anyone can do one, I'll be impressed.
I can do one pretty easily, after much practice. But that's it.