Microsoft US is sponsoring a competition for US high school students. It is called the Project Hoshimi Programming Battle. From the introduction at the MainFunction web site:

Your students will be immersed in an imaginary world where they are the only people who can save a person, a city, or even a nation. They will be asked to use their problem-solving and programming skills to save the day. The Project Hoshimi - Programming Battle brings to life the immersive world of the human body as students assist Professor Hoshimi and his crew of scientists and programming experts in a fantasy of life and death. The competition brings together an imaginative background story, comic-style graphics and programming challenges allowing them to compete online, with students from all over the US.   Student will devise strategies and write code for navigating a team of nanobots through life-saving missions and objectives through a map of the human body. We'll provide you and your students with the tools and resources; students will provide the smarts and coding skills; you will provide energy and motivation.  It will all come together in a fun, exciting competition in which students with great programming skills can win great prizes including an Xbox 360.

This is a chance for your top students to take on an interesting and fairly complex project. It also lets them see how their skills stack up to the skills of students all over the country. If you are looking for something to motivate your students to do something beyond what you are doing in class this may be just what you are looking for.