Air Wings is an awesome game for iPad, iPod Touch, or iPhone that my whole family loves to play, from my four year old to my teenagers and even my wife and I.
Using Apple's game center we can verse each other (well, up to four of us at a time) or we can compete against people online. There is a microphone feature so you can talk to each other while playing too.
Basically, you operate a paper or wooden plane and have to fly around collecting cool weapons like pencil missiles and shoot each other down.
It's easy to get the hang of and loads of fun, especially when you're playing your friends.
Each player needs their own game center ID to compete. They only take a minute or two to set up, and even little kids can get their own without setting up an actual Apple ID account.
There are some other cool features like the ability to AirPlay a Spectator Cam to your Apple TV while viewing the normal camera on your iPad. Get it from the iTunes App Store.