Just two slits

A further suggestion for restricting paths is to move from one to two slits. That is to have two gaps rather than a single gap, further modifying the model for propagation.(You might think, following on from restricted paths on the mirror, that a line of holes, or even an array of holes would be interesting. It would, and as an easy extension from what you're going to investigate here.)

The waypoints are again fixed by the slits, so use the model to explore the prediction about illumination at different locations for the detector.

You might prefer to work with triplets, rather than just the pair, in which case use:

Once you've seen that bands of brightness and darkness alternate, get some apparatus out and show this(it's the phenomenon of interference, central to the quantum world). Then celebrate a further unification.

The critical point here is that this alternating pattern of brightness and darkness still builds up even when the source is so dim that there is only a single photon in transit at once. Photons are just not like us – they really do behave as if they're exploring both slits at once (or if you have an array of slits, all of the slits at once). If the calculating we do is a reliable guide to how the photons are between source and detector then this is how they are. Not a stream of bullets. Not in any one place. The best we have is that they leave the source and we can predict how many will turn up at each detector by doing the calculations – here represented by exploring paths, defined by waypoints. Then summing the contributions to find the resultant, which predicts the relative brightness.