Here are some great things to try and mix it up a little:

Variation 1: Have the team play without using any part of their hand or arm to keep the beach ball in the air. Add bonus points for the most creative hits.

Variation 2: Challenge the group by adding additional beach balls to the mix.Two balls will definitely change the dynamic.
and furthermore...kids are rotten.....

Variation 3: Divide the group into teams, each trying to keep their ball in the air for the longest time, and watch the world of competition unfold. Be mindful not to set it up as competition, but watch how the teams default to it.