Project Euler is a simple and surprisingly good educational tool for a blend of computer science and math. Highly recommended.
You are given a problem (good examples:    ), go off and work on it in whatever programming language you like using whatever tools you like, and submit your answer (multiple submissions allowed). Simple, but surprisingly fun and interesting.
It's been around for a while (since 2006), and, though I've looked at it a few times, I only recently got addicted to it. It's not, as I first thought, just a series of interview-style coding questions, but a much more interesting set of deeper challenges in math that require programming to explore and solve. It's a great way to refresh on math and fun too.
Honestly, I can't say enough good things about it. I've blown hundreds of hours on some addictive video game before, addicted to the point that it occasionally interferes with work and sleep even, and this has the same feel. It's a great little educational tool and fun as well.
Definitely worth a look. Seems like it'd work for older teenagers too if you're looking for a summer project for a teen that already has some programming skill.