"There's an App for that!"
Let's spend a minute and look at some plain old fun applications. I love to play and have found many applications that can satisfy my urge to waste a few minutes playing. Besides it's a great way to bond with my teenagers. Some of my current favorites: Touch Physics (2.99)--there is also a free version but I had to upgrade to the full version because I liked it so much. I could argue that you have to know simple machines and laws of physics to play this game so it is indirectly educational.
Another challenging game is Bloons ($.99). Try popping the balloons using a limited number of darts. Each level is more challenging. Let me know if you can get past level 35--I've been stuck there for weeks.
Of course you can always find the classic games like checkers and connect four for free. Happy gaming!