For my CIID thesis project I conducted a series of small experiments with a pair of so-called Braitenberg-vehicles; small vehicles that have a single goal in life – either going towards light or away from it. In order to experiment I had to first design and build the vehicles. I decided to take the opportunity to create an easily reproducible kit, initally to share with my fellow students (or other interested parties) in case they wanted to play around with them. The kit consists of a set of schematics for laser cutting acrylic sheets for creating the robot base. All measurements are made to fit the most common servo motors, a standard sized Arduino, a micro-breadboard (easily available online) as well as a standard 9V battery cell for powering the bots. The two "antennas" are for mounting the photoresistors. I'd suggest cutting them in 3 mm acrylic sheets. After assembly you'll have to source some wheels for the servos, I used an old pair of Lego Technic wheels I had lying around.
It's up to you to give them some intelligence, you can check out the Wikipedia entry for inspiration. The basic idea is that the Braitenberg vehicles have no intelligence what so ever, but when people see them they suddenly perceive the vehicles as having different personalities because of the way they move and react. Despite the fact that my vehicles behaved the same through all my tests, they were perceived very differently by different people – the reason being that people project their own personality and understanding of the world onto them. It gets really interesting if you build two kits and swap the inputs from the photoresistors on one of them.
Here's the laser cutting template for making the kit shown above, remember to adjust line properties, etc. so they fit your machine – also, you might want to move things around a little, to save on material.
Note: Unfortunately I don't remember how long those screws were, but it won't be hard to find something that works. Alternatively you can use zip ties to hold the layers together.