The direct object is "soccer". It is possible to identify the direct object because it tends to inmediately follow the main verb, which is 'play' in this case. Also, a more effective means of identification is by replacing the word with the pronoun 'something'. Thus, if I paraphrase this sentence it will be: Scott and Maria aplay something. Always remember that a direct object is a syntactic element in a sentence, so it usualñy consists of more than one word, in this case the complete direct object would be: soccer with an old ball they found behind the shell.