One of the robots that you can build with the new NXT 2.0 kit is called ShooterBot - it uses the included Zamor Sphere shooter, the Ultrasonic Sensor, and the new Color Sensor. Add in using all 3 motors and the new rubber treads, and you've got a robot that you can program for all sorts of mischief.
Instructions for building ShooterBot are included in the NXT 2.0 kit's software - you can follow along with the instructions and create the more detailed program or use the nice new feature of simply clicking on the "sample program" icon to immediately have a smaller, simplified program ready for upload to your robot.
With the Ultrasonic Sensor added to the robot, your robot will scan its surroundings for any nearby "intruder" (less than 40cm). When it detects the object, it will emit a loud chirp (and the Color Sensor's LED will change colors) as a warning to back off! If the intruder is still there after a short pause, ShooterBot will let loose with the Zamor Spheres.
This robot has PLENTY of room for expansion, too - you've got two Touch Sensors included with the kit, which means you can add some additional protection to the robot - instead of waiting for the intruder to move, if a touch sensor is triggered, you could program the robot to immediately turn in the direction of the triggered Touch Sensor and begin shooting.
Other options include using the Color Sensor as a way to distinguish colored targets that you place around your room - have a friend build his/her own ShooterBot and have a contest to see who can find and knock over the most targets of a specific color.
ShooterBot is the first robot in the RoboCenter portion of the NXT-G software, but it may not be the first robot you build... other options include the RoboGator, Colour Sorter, and the new and improved Alpha Rex.
More details on these other NXT 2.0 kit robots coming soon...