I've been told by many teachers that my NXTurtle pet is beloved by children. That robot is described in detail in my book Creating Cool MINDSTORMS NXT Robots. Apart from walking and turning, the turtle can retract the head inside its shell. Since it is getting 3 years old by now, it was time to redesign it with the NXT 2.0 kit, giving birth to NXTurtle 2.0!
This new pet has an autonomous behavior, and a crazy new feature you will discover watching the video till the end.
You can download the building instructions and NXC programs for this robot from here.