Animal charities in abundance, but the one I would go for is GoMad which helps people, some of whom live in terrible poverty. GoMad collect everything, food, clothes, toiletries, many things which you and I might throw away, but they can find a use for. They organise fund raising events, have stalls etc. I don't think there are any other charities run by Ex pats which di this work, do help is sorely needed.
if you get a copy of the Costa Calida Chronicle, or the Camposol journal , you can find out more about the area and volunteering.
I think they are also online.