Motion Tennis (free for a limited time) is a clever interactive tennis app that has some attributes of the Nintendo Wii for playing motion-based games. You'll need to have a 2nd or 3rd generation Apple TV and the Motion Tennis app running on your iPhone (4S or later) in AirPlay Mirroring mode. You can play at several levels against the computer opponent, or play with a second iPhone opponent.
You start with a wide view of the tennis court, your opposite at the top of the screen, while you are near the bottom in a 3D perspective. By moving your phone you can simulate forehand and backhand shots, and it's easy to slice and lob. The sensors in your iPhone make it all work. Like a tennis pro, you have a choice of grass, clay or synthetic courts.
I was a bit skeptical, but the game works pretty well. There is a little lag on your first serve, but after that the game stays in sync with your motions. I tested the app by doing nothing when a ball came my way, and it zipped right by me giving my opponent the win.