Buying an Xbox for my son. What games should I be buying for myself?

For Christmas my son is getting an Xbox with Kinect. He's got to sleep some time so I'm wondering what games are the must-haves for someone new to the xbox world. Also I'm old so don't want to just go online and have my ass handed to me by kids who play online 8 hrs/day...

Edit: Thanks for all the responses. Gonna have fun!