Maybe your local park isn't much more than grass and trees. If your community's play space could benefit from features like a playground or a basketball court, organize a fundraiser in your neighborhood to get money for these projects. Be clear about what you want to do and how much money you'll need to do it -- people are more likely to give to a cause when they know exactly what they're funding.
Here are a few ideas to get you started: Throw a block party to generate excitement for the project, put an ad in your local newspaper or magazine, post flyers around your neighborhood, or create a blog where you can track your progress and post pictures of your work (make sure to include before and after shots)! You just might be surprised to discover who's interested in partnering with you.