The color camera in his nose. Ugobe may have originally planned facial recognition, but currently Pleo can only use it to "get a feel of his surroundings."
He also sports binaural hearing with a microphone on each side of his head. The microprocessor compares the sounds received from each "ear" and allows him to work out the direction and distance of sound sources.
An infrared interrupter in his mouth lets him know if you have placed something in there. There is also an IR communicator which allows him to talk to other, like-minded dinosaurs.