The prizes are cool and all you need to do is make an app. In fact, for Apptivate at least, your app doesn't even need to be complete to win!
Can't think of an app to make? No worries, just ask yourself: what site do you like and would you like to bring it to Windows 8 or WinRT? Then ask, does it have an API or feed or enough of something to make a usable app? If the answer is Yes to both of those questions, you have your app.