I believe admission staff to the schools of that nature are looking for some real individuality. They're looking for passion. An app they'd like would be one that you were like, "Hey, I have this need, there'll be a lot of apps for it on the app store..." few searches later, "Holy crap, no one else has actually made this yet. I absolutely have to learn how to program so I can make this app that no one else has."
So, I'd suggest thinking about the kinds of apps that you'd like to buy, except they're not for sale because no one has made them yet, and then go make that.
For me... I made an agenda program capable of learning my class schedule based on when I entered things into it. That was enough to blow away the Eagle Scout board, but I suspect their standards are a good deal lower than Stanford's. I've not released the app ever, because I never polished it enough to do so, but I demonstrated it in person on my own iPhone.