If you want something challenging and fun to do on your iPhone while waiting in holiday lines and in busy waiting rooms, try Kotaku's list of what they deem are the best game apps for iPhone. As the intro to their article says, sometimes going to the App Store and just looking around can be an exercise in frustration:
"You might not want to just plunge into the App Store. It's a jungle, and it is full of bad games. Here, Let us help you. After all, the games at the top of the App Store charts might not always be the actual best games for the iPhone. We think the ones below are a great place to start."
See their list of the best games for iPhone here. If you think they're missing some games, feel free to weigh in with your favorites in the comment section.