You will need a number of ping pong balls (at least one per student) and a basket, collapsible bucket, or similar small container.
Place the bucket in the center of the table. It can remain here for the duration of the class, unless you think it will be in the way or a distraction.
Give each student a ping pong ball. On the count of three, everyone has to bounce their ping pong ball off the table and attempt to get it into the bucket.
Whoever bounces their ball into the bucket receives one point.
VARIATION: Each student gets three, four, or five ping pong balls each (NOTE: they will need to share the balls). Students take turns trying to bounce as many of their balls into the bucket as they can, receiving one point per ball that lands in the bucket. Additional points are earned for multiple bounces.
VARIATION: Make it increasingly more difficult over the course of the lesson to bounce balls into the bucket by increasing the height of the bucket (eg. place one bucket on the table upside-down, then place a second bucket on top of the first).
WARNING: Do not make the game overly complicated. This is meant to be a fast, easy to understand game that injects a bit of fun into the class without detracting from the lesson.