A simple tennis game between two iPhones nearby each other using Bluetooth or Wifi.
When connected via the connection screen, either player can serve as many balls as they like by tapping the screen to create a ball, then hitting the ball by moving the bat to the ball. When a ball reaches the net it will appear on the other device as if it were hit over the net - the other player then moves their bat to hit the ball back.
A ball passing your baseline gives your competitor a point. First to 50 wins. The scores can be reset at any time by a player touching their own score.
If the other iPhone disconnects from the game their score on your screen will turn red. You can touch their score to bring up the connection screen and reconnect when they return.