Using a beach ball, start hitting it around and trying to keep it off the ground. Then challenge them to keep it in the air for 20 hits, or 30 hits, etc. Encourage them to develop some strategy such as establishing zones etc. to try to keep the ball in the air for as many hits as possible. You can impose more challenging rules such as limiting the number of consecutive hits any one person may have or limiting hits to feet only etc.
Keep count and try to beat your highest score. You can also break into two groups and compete.