I second this, and add that if you have an area of expertise, you will have more impact if you utilize that expertise. Some examples:

• If you have access to a commercial kitchen during off hours, you could cook food for the poor.
• If you are good at socializing, try meeting with local political groups, influencing election candidates, or running for office.
• If you are a software engineer, volunteer on an open-source project related to a cause that you care about.

And if you don't want to spend effort, you can always donate to a cause.