It doesn't have to be a business idea! Just an interesting idea that would otherwise require a lot of compute or storage, or something else provided by AWS.
chrischen. I am also facing similar situation. So, which out of all the suggestions did you found the most interesting one ? And what have you actually planned doing with it ?
What's your background? Are you looking for business idea that can be built upon your existing site, or are you looking for something completely new?