I've got the Mindstorms NXT for two months now, but haven't got around to making the various models yet. This is the 'Hello World' startup model, there're many more in the included CDROM and website . Since Lego is to be played as 'sandbox', one can find inspiration from hundreds of other projects too. More lesson plans here and here.
And I've just realised there's also a handphone Bluetooth application. It can be used to run programs and collect data, and drive the thing around using the handphone joystick. My first remote control car!! Run, robot, run! ;p