I've been experimenting with some wheels I found on a different, lessor, robot. They are about the right size, and they give a Hull Pixelbot a nice "off-road" feel. They make turning a bit more difficult though, as the thick tyres are very grippy.
I've also been playing with printing "light pipes" inside the shade for the pixels. The idea is to give better defined points for a camera to track the position and orientation of the robot. But I think the results are quite artistic too.