I highly recommend the Giving Back: Introducing Community Service Learning (Improving Mandated Community Service for Juvenile Offenders) Curriculum. It is freely available online.
The curriculum introduces the process of service learning for teachers/community service personnel interested in guiding students through the development and implementation of their own service projects. In addition, the curriculum includes a list of ten "Done-In-A-Day" service projects.
Though the curriculum is intended to teach civic responsibility to students who have committed criminal offenses, it is suitable for use by all children (though most of the projects are suited to secondary level students).