We talked semi-seriously about building a simulation of a motorway in occam, but it'd also be fun to build a train set, with track segments and trains being interacting processes. This'd work particularly well with a fancy visualisation in which each process sent drawing commands to the screen, and where processes could be connected up and moved around interactively.
For both the motorway and train set models, cars could detect things around them by registering with overlapping blocks: we'd have two sets of blocks offset by half a block, so that if a car registers with the two at its current location it's guaranteed to be able to see half a block either way. This could provide very efficient 3D collision detection for TUNA simulations too.