Real-time Groupon. All sorts of businesses lose money due to last minute cancellations - give them a platform to advertise discounted slots in the near future.
That's actually been tried by Groupon in the past... It wasn't successful largely because it's hard to get merchants to adopt that way of thinking and also because people don't walk around with the Groupon app open all the time.
Doesn't really need an app, just needs a phone number. Even if your initial marketing list for a business is every customer they've ever had, you're still starting with something.
That's an awesome idea. You can just create an offer in seconds and publish it. And have an app track you based on locations or checkins, remind you once you are in the restaurant.