Head over to eBay and pick up the Meccano Meccasaur Programmable Robotic Dinosaur for only $39.98 from the Toys R Us store! This set comes with 715 pieces that you can assemble into a 3 foot long programmable T-Rex! It responds to petting, answers yes/ no questions, and stomps around.
Keep in mind that this is quite the project and will probably take quite a while to put together! It might be a nice project to work on with the kiddos here and there.