NASA have collated a enormous collection of Robotics activities to use in the classroom. Activities are designed for a variety of different age groups and abilities.
It looks like it has not been updated for a few years, and the majority of activities are designed for use with the older RCX, but there is no reason you couldn't modify them to suit the NXT.
If you're a teacher, have a look through a few (there are 300) and let us know in the comments which one is your favourite.
My favourite is "Junkyard Wars: Mechanical Monsters" Date Posted: 12.27.04