In the Temple of Heaven park. The women were doing an exercise using a paddle made out of leather and a small hard plastic ball. They moved in synchrony without dropping the ball on the paddle. Many people in the park played a game passing the balls back and forth, often moving the ball behind their backs gracefully before passing it on. I tried it once and it was harder than it looked. Of course the people who let me try wanted to sell me the paddles for 60 RMB--I checked out the actual price later and it wasn't that much of a markup. Now, I wish I'd bought one..
This temple is where emperors in China used to pray for divine guidance.