You can first write it in a happy tone, to have a longer essay. You may express how you and your family were enthusiastic on your picnic beside the windy and beautiful beach. Then, you noticed that you were not enjoying yourself for you can see many rubbish gathered by the seashore.
Therefore, you suggest ways on how to make the environment a better one by launching a cleaning up activity. That way, society will not only have a neat place but also can spend their time with their loved ones on the beach.
Hope this helps.