I would also recommend a cause over a candidate.
moveon.org and other political groups have resources where you can either locate a cause or candidates that campaign for specific issues.
Others’ suggestions for HH and Sierra Club and Surfrider are also good ones.

That being said, my other recommendation would be to suggest the child help with the campaign of some VERY local candidates–your local school board. Your kid will know immediately where each candidate stands on issues close to their world of experience. The kid will be able to speak to the topics at hand immediately and know whether the candidate has students’ best interests at heart.

Best of luck. I think it’s great that you are encouraging civil participation at this age–there are so many jaded teenagers who don’t get involved. It’s great that you are fostering their participation.