1) micro switch/limit switch - if sensitive enough to be triggered by a ping pong ball, probably not.
2) piezo touch plate, can be quite sensitive to impacts.
3) break beam sensor, triggers as the ball falls through the beam.
4) light sensor under the ball - the ball darkens it as it falls onto it. Use a fast one such as an LDR.
5) ultrasound sensor - can detect an object as it approaches or passes by. Maybe not enough reflection from a ping pong ball.
6) laser range finder - to detect the ball as it falls down.
7) camera - such as the Pixie.
There are probably more options.