Well you could baby sit like SavannahLovesya suggested. But I would suggest NOT working at a grocery store (I don't even think you could at 13 anyway) because my brother did and he said people were warning him that customers have told them to get in their car. which is unlikely it will happen, but someone your age should not take risks like that.
Try washing cars. That is not an exclusive for boys. Girls in Bikinis get a lot more customers than boys do. Maybe team up have the boys do the washing while the girls stand at the street in bikinis holding signs?